What is Knock-and-announce?

Legal Definition
Knock-and-announce, in United States law criminal procedure, is an ancient common-law principle, incorporated into the Fourth Amendment, which requires law enforcement officers to announce their presence and provide residents with an opportunity to open the door prior to a search.

The rule is currently codified in the United States Code, which governs Fourth Amendment searches conducted by the federal government. Most states have similarly codified the rule into their own statutes, and remain free to interpret or augment the rule and its consequences in any fashion that remains consistent with Fourth Amendment principles. A state's knock-and-announce rule will govern searches by state actors pursuant to state-issued warrants, assuming that Federal actors are not extensively involved in the search.
-- Wikipedia
Legal Definition