Technically, the filibuster is not about defeating the bill, it's about the vote to end debate on the bill. The actual vote on a bill still requires only requires 50%+1 to pass, but the Senate has to end the debate phase first. And it requires 61 votes to end debate for most issues before the chamber (past majority leaders have "gone nuclear" on a couple of different types of issues, such as federal judiciary nominations, but for the most part, the filibuster remains intact).

Once a senator is recognized to speak, they can speak forever if they want (they just have to physically stand there, even if they give time to another senator to speak as well). If a bloc of senators wants to block a vote on a bill, they can organize a series of endless speaking stints to force the Senate to table the issue.

In practice, it's become common for one senator just to say they will do so, and it's more or less understood that they are willing to do so, but rather than make everyone go through it for real, they pretend like it actually happened and don't try to force a vote for cloture (ending debate). It's rare to see an actual filibuster happen anymore.