A filibuster is any “attempt to block or delay Senate action on a bill or other matter by debating it at length, by offering numerous procedural motions, or by any other delaying or obstructive actions.” U.S Senate website
To better understand the filibuster, it’s necessary to understand how the Senate passes a bill. When a senator or a group of Senators introduces a new bill, it goes to the appropriate committee for discussion, hearings, and amendments. If a majority of that committee votes in favor of it, the bill moves to the Senate floor for debate. (Brennan Center)
Once a bill gets to a vote on the Senate floor, it requires a simple majority of 51 votes to pass after debate has ended. But there’s a catch: before it can get to a vote, it actually takes 60 votes to end debate. This is why a 60-vote supermajority is now considered the de facto minimum for passing legislation in the Senate.