Senate Democrats plan a vote to change the filibuster. So what is it?

In a video series originally published in 2017, NPR explains the filibuster – a 60-vote threshold for most legislation in the Senate. We're bringing this explainer back now that Senate Democrats and President Biden are talking about changing the rule in an effort to pass a set of voting rights bills.

The filibuster hasn't been immune to change — though doing so is exceptionally controversial given the power it wields. It used to be a rare and powerful tool for lawmakers when they wanted to hold up the normal course of business.

But as rules...