What is a no knock warrant?

The police obtain a warrant from the judge under the premise that if they were to alert you before entering your home it could pose a physical risk to the officers and/or you would likely destroy evidence before they could get inside.

A "no knock" warrant allows the police to enter a building, residence, etc. Without notifying the occupants that they are coming in before entering. Basically, they can smash down the door and walk in.

It is pretty much exactly what it sounds like, in that it allows the law enforcement agency enforcing the warrant to skip the "knock" in "knock and announce" requirement of executing a warrant. The purpose of such warrant is to avoid the possibility of destruction of evidence or escalation of violence.