If you’re on a website and you click a button and something happens other than going to a different page or downloading a file, it’s almost always because of JavaScript (JS)

It’s a programming language that’s built into most browsers that allows for interactive functionality. It’s implemented in a way that typically protects the security of the computer, because it doesn’t have access to the files on the computer or other sensitive things.

Because it can’t directly check a lot of things, browsers often provide a way for people writing websites to check what capabilities they can use.
This means there’s a way to ask "can I do 3d graphics using this particular feature?”.
The intent is so you can let your website handle computers that can’t do what you need, rather than just trying and crashing.

Sufficiently determined people can ask for all of the various clues that are provided, and use that to make a pretty unique identifier for the computer, which they can then use to track you around the Internet.

Various ways of fixing this have been proposed, but they all fall short one way or another.
Either by making the web far less interactive than people have come to expect, making tracking easier but at least they’re not misusing the data, or requiring an unrealistic amount of reworking how the entire web works.

JavaScript, code that websites can run in your browser when you open them