The central holding in this case was that the exclusionary rule applies to the states and not just to the federal government.
For decades before the Mapp decision, the exclusionary rule had applied to the federal government. This rule says that any evidence that is acquired as a result of an illegal search or seizure must be excluded from a trial. It cannot be used against the defendant. This rule was meant to give teeth to the Fourth Amendment. However, the Bill of Rights was only applicable to the federal government. States did not have to abide by it.
In the 20th century, the Supreme Court started to use the 14th Amendment to “incorporate” the Bill of Rights and make it apply to the states. Mapp was a case that incorporated this particular aspect of the 4th Amendment. In Mapp, the Court held that evidence that was obtained illegally could not be used in state trials any more than it could be used in federal trials.
Thus, the central holding of this case was that the exclusionary rule should apply to the states as well as to the federal government.