American courts use the exclusionary rule to deter police officers and other government agents from abusing constitutional rights. According to the rule, courts will suppress evidence that the government obtains through unconstitutional conduct—often an unlawful search or seizure. Suppression means that the evidence in question will be inadmissible for most purposes in the defendant’s eventual trial. If a judge suppresses crucial evidence, the prosecution may have no other choice than to dismiss charges. (See What is a motion to suppress?)
The exclusionary rule applies to evidence that's a direct product of a constitutional violation. It also comes into play when such a violation leads less directly to incriminating evidence.
Suppose officers, without reasonable suspicion or probable cause, stop a man walking down the street. Though he’s done nothing to deserve it, they search him. They find in his pocket a scrap of paper saying, “Drugs are under garbage bin at 123 Jones Street.” The officers go to the address, peek under the garbage bin they see there, and find a bag of various narcotics. Both the note and the drugs were a product of an illegal stop and search, so neither will be admissible to prove the man’s guilt.
For more information on the exclusionary rule, including exceptions, see Fruit of the Poisonous Tree and Police Searches and the Good Faith Exception. Also see Statements Obtained When Police Violate Miranda.