I've not come across Framer before, but I got my head around it in about 10 minutes. There's an easy route to leverage Photoshop mockups, but you can do it from scratch without too much trouble (and there's a plug-in for Sketch if you–like me–avoid Photoshop).

Many people already prototype in the browser. It's simple and quick. But while html/js/css/jquery gets a lot done it has some downsides:

It can get pretty complicated mixing all the different technologies

It can be hard to get the pixel perfect control you want

It's not always performant, especially on mobile

It's pretty far from how it will be actually implemented if you prototype for native

Framer tries to solve some of these problems by providing a very lightweight framework modeled after larger application frameworks. The basic idea is that you only need a few simple building blocks like images, animation and events to build and test complex interactions.

Some useful additional background also in this case study of building Potluck.