The Fourth Amendment to the US Constitution protects you against unreasonable searches and seizures. Generally, this means that the police or other law enforcement agents can't search your property and take your things simply because they don't like you or just because they feel like it. They must have a good reason before they can search your home or office and seize things, such as contraband or evidence of a crime. When the Fourth Amendment is violated, any evidence that can be traced to the illegal search or seizure is fruit of the poisonous tree and can't be used against you.
Excluding or "Suppressing" Evidence
The tool used to enforce the Fourth Amendment's protections is called the "exclusionary rule . Generally, unless one of the warrant exceptions applies, the police and other government agents need a search warrant before they can search your home, office, luggage, or even your pockets. A search that's made without a warrant, or in violation of a warrant, is considered an unreasonable search. And, under the exclusionary rule, any evidence that's obtained in violation of the Fourth Amendment - that is, through an illegal or unreasonable search, seizure or even an arrest - can't be used as evidence against you to convict you of a crime. The evidence is "excluded" or "suppressed."
For example, say police officers get a search warrant that authorizes them to search your backyard for growing marijuana plants. They don't find any plants in your yard. However, during the search, an officer enters your basement and finds bundles of packaged marijuana and seizes them. You're then arrested and charged with possession of illegal drugs. Under the exclusionary rule, unless the prosecution can convince the judge at trial that the officer was justified in entering the basement, the marijuana will be excluded from evidence and can't be used against you because the search and seizure was illegal - the police were authorized to search only your backyard.
Fruit of the Poisonous Tree
The "fruit of the poisonous tree" doctrine goes hand-in-hand with the exclusionary rule. In fact, it takes the protection given by the exclusionary rule one step further. Under this doctrine, the state can't use evidence at trial against you if it was discovered through other evidence that was obtained in violation of the Fourth Amendment. The "poisonous tree" is evidence first seized or discovered through the initial violation of the Fourth Amendment - an illegal search, seizure, or arrest. The "fruit" is any evidence that's uncovered later because of information or evidence obtained from that illegal search, seizure or arrest.
Both the exclusionary rule and the fruit of the poisonous tree doctrine are designed to curb or deter misconduct by police and other government agents. They work on the assumption that the police will do their best to make sure that they don't violate the Fourth Amendment if they know that they can't benefit from any evidence found or uncovered as result of an illegal search or seizure.