So cloture is a fancy word for ending debate on a bill and voting for it.
Originally any Senator could say they wanted to continue debating the bill. But then they had to actually “debate” it. So if there were 8 or 10 Senators who didn’t want a bill to pass they could organize: “you debate from 8-10, then I’ll go from 10-12, then Billy Bob from 12-2, etc.
If it was just one guy then he had to stand there and talk until he physically couldn’t do it any more. Because as soon as he gave up and sat down they could “invoke cloture” which ends discussion and means they can take a vote.
This is where the stories of Senators reading from the phone book came into play. They’d run out of even marginally thoughtful things to actually say about a bill and resort to “here’s a list of people who will be harmed by this bill: Abrams, Andrew & Sheila residing at 123 Apple Tree Lane; Abrams, Bill… etc.” Because they literally had to keep talking.
And they could only be considering one bill at a time. So while they were filibustering they were basically holding the Senate hostage.
It was an extreme measure only used very sparingly about issues that Senators were especially passionate about (like preserving white people’s right to not have to be around black people… very very important stuff). But, however demented the reasons for utilizing the filibuster, it WAS used sparingly. (And obviously I was being sarcastic about the “very very important stuff” but it was important to those particular Senators.)
And I’m not exactly clear on the precise progression of the rule changes. But they are now permitted to halt debate on Bill A and start debate on Bill B. So it no longer prevents Congress from doing anything else like it used to. And they no longer have to physically stand there and talk for hours and hours and hours.
Strom Thurmond infamously filibustered the Civil Rights Act of 1957 by himself for 24 hours at which point he could not physically continue. Cloture was invoked and the Senate voted on (and passed) the bill.
By the time the Civil Rights Act of 1964 came along it took 34 Senators, not just one, to filibuster. But the Southern Senators got together and organized a filibuster that lasted 60 days. But that filibuster too was broken and cloture invoked and the bill was voted on and also passed.
At some point between 1964 and the 1990s it went from 2/3 necessary to invoke cloture to the current 60%. Thus the number of Senators needed to successfully execute a filibuster increased from 34 to 41. But everything else about a filibuster like actually having to stand there and talk, and becoming a pariah because you prevent literally anything else from happening while you’re doing it, became easier.