|This page in a nutshell: If an otherwise reliable source attributes information to an unreliable source then that information is likewise unreliable.|
Fruit of the poisonous tree is a legal metaphor in the United States used to describe evidence that is obtained illegally. The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well. The metaphor can also be applied to citing sources. If information gained from a reliable source (the "fruit") traces back to an unreliable source (the "tree") then that information is unreliable as well.
Anyone on Wikipedia must be able to check that the information comes from a reliable source. If the information found on a reliable source traces back to, is attributed to, or re-printed from an unreliable source then the information did not come from a reliable source, but originated from an unreliable source. Information that originates from anything other than a reliable source or was not independently verified by a reliable source is original research. Furthermore, any speculation or rumor from reliable sources is not appropriate for Wikipedia. Sources that rely on Wikipedia may not qualify as reliable either.
Most sources attribute their information to someone or something, either at the end of the article or in the body of the article. Watch for keywords or phrases like "reported", "according to" and "via". Avoid sources that trace back to unreliable sources. If you are not sure if the source is reliable then please read WP:IRS or check the reliable source noticeboard and create a topic if no topic already exists.