It's an open source document preparation system. Essentially a typesetting language that produces good looking pdfs. It's the de facto standard for e.g. scientific publications.

It's not as simple as a "what you see is what you get" editor like Word, but a lot more powerful. It can be relatively simple to get started with by messing with a template on Overleaf.com, but it has quite a learning curve if you want to typeset documents from scratch.