The Senate is required to follow certain procedural steps in passing legislation. When a bill is brought to the Senate floor, any senator can bring things to a halt by speaking for as long as they wish, effectively delaying a vote to end debate on a bill. The Senate can vote to end debate by invoking cloture — another procedural step — with a three-fifths majority, or 60 of 100 senators. So any bill that has the support of at least 60 senators is, in effect, filibuster-proof, and the Senate can quickly move on to the next steps leading up to a final vote. But without 60 senators (remember, Democrats only have 50 right now), the filibuster cannot be broken.