On Wikipedia, an essay is a page written in Wikipedia project namespace describing the processes on Wikipedia at the point-of-view of one or more users. Essays can be written by anyone and can give instructions or advice, can be long or short, serious or funny.
There are currently thousands of essays, and this number is expected to grow. Essays vary in popularity and how much they are followed and referred to. As with WikiProject pages, information pages and template documentation pages, essays have not been formally approved by the community, thus generally have limited status during deliberations (see WP:Local consensus for details).
Essays, WikiProject pages, information pages and template documentation pages, do not automatically become policy or guidelines just because they are written. All it takes is one person to write an essay or information page or create a template and its documentation page and there it is. Unlike a policy or guideline, which requires a clear consensus before it can take effect, and will be in a proposed state until then, an essay does not need consensus to exist; it just is. Rather, following the instructions or advice given in an essay is optional, assuming that this choice be made wisely.
Some essays at one time were proposed policies or guidelines, but they could not gain consensus so they were converted into essays. Other pages that began as essays later became policies or guidelines or tagged as a supplemental essay.
No. Essays and information pages do not override existing policies and guidelines. They are just an addendum to existing ones. Essays do not serve the function of creating new policies or guidelines or rendering existing ones meaningless. As policies and guidelines can have multiple interpretations, essays serve to show various interpretations of policies and guidelines that are already being somehow followed.
Essays and information pages are not policy or guidelines that must be followed, but they are likely worthy of consideration. An editor who takes the time to write an essay probably understands the project namespace well enough and has enough knowledge and experience in editing Wikipedia that the essay has been written in good faith. Essays usually are based on reform, gaps one sees must be filled in, or other improvement viewed as necessary in Wikipedia's procedures. An essay may also provide advice on how to apply a particular policy or guideline to a specific situation. Essays edited by multiple editors might be given extra consideration as this is a clear sign that the viewpoint exists from more than just one editor.
The answer is you can't. No essay or information page has any special status of having more worth than another. You can, however, see how popular a page is.
Essays and information pages, like all other Wikipedia pages, can be edited. They can be expanded, reduced, modified, merged, split, replaced with random obscenities or even deleted if deemed necessary (via WP:MfD; this is extremely rare). Few essays are protected in any way, allowing them to be edited by IP editors too. All the same guidelines that apply to editing articles apply to essays. Essays are not owned; no special permission is needed to edit them. Good judgment and understanding of their meaning and area(s) of coverage is strongly encouraged.
Unlike policy and guideline pages, bold edits intelligently made to essays and information pages without a discussion are less likely to be reverted.
Expansion to essays and information pages is highly welcome. It is strongly encouraged that if you want to create an essay, that you attempt to see if an existing essay essentially giving the same message or one in which your proposed message can be inserted already exists. Some new essays, however, are created resembling other essays as POV forks, which is acceptable in essay writing.
Currently, many essays are orphaned. It is important if you create or improve an essay that you try your best to see that it is not orphaned and it does not form a walled garden with several other essays.
Essays can be linked from other essays and information pages or other relevant policy or guideline pages, since they often pertain in one way or another to policy and guidelines. See Wikipedia:Drawing attention to new pages for instructions on how to make links to your newly created or discovered essay.
A question that has often arisen is whether or not an essay is valid for making a point in a deletion discussion.
The outcome of a deletion debate is determined not by votes, but by Wikipedia policy and guidelines. So where does an essay figure in?
Like policies and guidelines, essays are likewise useful in making a point for your cause. But if you do, simply stating "Keep per [[WP:ThisEssay]]" or "Delete per [[WP:ThisEssay]]" will not help your cause. Your point-of-view will have a better chance of being factored into the outcome if you state the reason why you believe the essay you cite matters. You may even use an essay as a rationale to propose an article for deletion if you give a good reason. WP:HAMMER is a classic example of this.
If someone has cited an essay (or part of) as a reason for their cause, someone else might state that what they cited was "just an essay" and therefore is meaningless in the discussion. Truth is, essays are not meaningless and will be factored into the outcome if expressed well. If you want to counter an essay cited by someone else, the best way to do so is to cite another policy, guideline, or essay along with an explanation as to why your opinion is more worthy.
For a list of good essays to use in a deletion discussion, see Wikipedia:List of policies and guidelines to cite in deletion debates#Useful essays (and parts of essays).