This page in a nutshell: If you say something troll-ish to someone "feeding a troll"... Don't. Just don't.
This troll is ready for feeding; note the collection of utensils on his belt.
One day, a person was at a Talk page "feeding a troll". A random editor walked in, and started eating the person's "troll food". The troll then ate the editor.
On talk pages, there are trolls who "troll" people. When people argue back, that's called "feeding the troll". However, sometimes someone (who may or may not be another troll) will join the conversation out of "nowhere" and will probably start insulting either the troll or the other person. The troll will probably start trolling them both now and the other person will go "eat" the troll's "food".
Remember, You're making people unhealthy when you eat "troll food".