In essence, these are frameworks that wrap a view library (e.g. React, Vue, Svelte, etc.). They provide a bunch of the things we end up adding to an app (routing, data fetching, organization patterns, etc.) and provide some nice things both for DX and UX out of the box. So you're able to hang out in a familiar place (e.g. using React or Vue or whatever) and still get all these nice things that are typically pretty annoying and time consuming to set up and get right.
These tools have really seen a surge in popularity the last couple years and it's only going to continue. They're getting better super fast and it's pushing all of them forward (as well as the view libraries themselves). In fact, the new React docs removed their recommendation for Create React App and instead recommend using one of these frameworks. And a core React dev went so far as to say you should be using one of these frameworks and not vanilla React (there's a tweet thread about this, but I can't find it anymore).
Even if you don't have a ton of experience/exposure to these frameworks, I'd highly recommend using one them the next time you are reaching for React or Vue or <insert-JS-view-library>. They all have their quirks and things to learn and such, but the curve is pretty darn flat in my experience. And it's just so nice to not have to worry about all that set up we're so used to doing.