Hands down best Star Wars production Disney has made since buying Lucasfilm.

Criminally based series.

Glad people are finally paying attention to it
It deserves it

I only watched the 6 movies and 2D Clone Wars. What is this one about? Do I need to watch everything else?

This is about how the rebellion started
You do not need to watch any other Star Wars to understand it

In fact, it's so different in tone and style from the other movies, it doesn't even seem like it's Star Wars
It works perfectly fine as the story of a rebellion against an oppressive regime even if you don't know Star Wars

What were they producing at the prison exactly? Some AT-AT parts?

Death Star parts
Shows in post credit scene