In the simplest cases involving the exclusion of illegally obtained evidence, the items the defense is trying to suppress, such as drugs found during the search of a suspect's pocket, are direct, or primary, in their relationship to the police action. Thus, if the police have acted unlawfully, the evidence must be excluded from trial.
Many times, however, evidence is derivative, or secondary, in character. For example, an illegal search may turn up a key to an airport locker where the proceeds of a bank robbery are being kept. Or a coerced confession may reveal the place where a suspect hid a murder weapon. Or an illegal tap of the defendant's phone may reveal the whereabouts of a person willing to testify against the defendant. In such cases it is necessary to determine whether the derivative evidence is fatally tainted by the initial or primary government illegality. To use the phrase coined by U.S. Supreme Court justice Felix Frankfurter in a 1939 case, in these instances, the question presented is whether the challenged evidence is the "fruit of the poisonous tree."
Kamisar, Yale. "Weighing Poison Fruit." Am. Law. 25, no. 10 (2003): 65-9.