This blog post is pretty buzzword-heavy, but Penpot is a legitimately great tool. It’s used for UI design and layouts. I’ve seen a couple of open source projects use a self-hosted Penpot instance for working on and discussing new designs.

Figma would be the most popular, proprietary example of this type of tool. I’m not aware of any open source competitors besides Penpot.

edit: It’s like Google docs for web page layouts or app layouts. The animation on their homepage is probably the best way of showing what it does.