The exclusionary rule states that evidence obtained by illegal means is inadmissible in court. In other words, it is excluded from being presented as part of the prosecution's case (hence the "exclusionary" rule). An example would be evidence obtained by police without a search warrant.
Such evidence was rejected by the Supreme Court in its landmark ruling in Mapp v Ohio (1969), which extended the exclusionary rule to cover cases involving states—in this case, the state of Ohio—as well as the Federal government. The Court ruled that the plaintiff in this case had had her Fourth Amendment rights violated by having her property searched without a warrant. All evidence obtained in that unlawful search was, therefore, unlawful, and in a 6-3 majority, the Supreme Court overturned the plaintiff's original conviction.
The so-called "fruit of the poisonous tree" principle basically says much the same thing as the exclusionary rule: that evidence obtained in an illegal fashion is tainted. According to this legal metaphor, if the source of the evidence—the "tree"—is tainted, then the "fruits" of that tree—anything gained from it—is also tainted. As with the exclusionary rule, the "fruit of the poisonous tree" principle is designed to deter police officers and other law enforcement agencies from obtaining evidence illegally.