For example, Flutter uses it to draw its UIs and Chrome uses it for almost everything, including rendering text parsed from HTML. Sublime Text, Firefox, Xamarin and many other projects also rely on Skia for the same sort of thing.
However, unlike, let's say, Qt or GTK+, Skia does not provide already done widgets (i.e., drop-in buttons, windows, text inputs etc.). It's like the fundamental building block on top of which you can create your buttons, animations and everything else and then display it on screen.
Now, CanvasKit is Skia ported to the browser via WebAssembly. Since Skia is a mature project and very performant (as you can guess from who is using it), it's now an alternative to Canvas API and DOM to render things on websites and will also let existing native applications to be ported to the web more easily.