A filibuster is a Congressional procedure in which members of the Senate can stall a bill from being voted on. Members of the Senate are entitled to as much time as they want in order to discuss a bill. This can result in a small group of Senators stopping a bill from voting.
These Senators can take up enough time so that the bill expires or Congress ends its session. This tactic makes it so that a select few people can almost completely shut down Congressional proceedings on a new bill.