Fruit of the poisonous tree is a legal doctrine according to which any secondary evidence obtained indirectly through illicit means is inadmissible in court.
Examples of such sources include evidence gained through eavesdropping, illegal wiretapping, coercive interrogations, unwarranted searches, or improperly conducted arrests. Information obtained from those sources is inadmissible according to the law of exclusion. The fruit of a poisonous tree doctrine is an extension to that law.
Here's one possible scenario: If an illegally conducted electronic discovery (e-discovery) search yields information about corporate espionage, that information cannot be presented as evidence in court, according to the law of exclusion. If other information learned through the search leads investigators to search a public location, where they uncover evidence of another crime, the doctrine of the poisonous tree disqualifies that evidence for presentation in any legal action.
Both the law of exclusion and the fruit of a poisonous tree doctrine were created to discourage law enforcement officials from using illegal activities in efforts to obtain evidence.