yeah basically. it can be a number of thing, but yes, large amounts of back story for characters and the world and everyone's motivations; maybe some set design principles, etc. A lot of things the audience will probably never see, but can be referenced by writers to keep things cohesive.
Some of them are pretty easy to find, and you can see how they helped the shows be better. They sometimes have little bits of early scripts, maybe the pitch to the studio, and almost always where we are and where we are going.
They are useful since, in TV, when you write many episodes, you have a lot of writers and a lot of moving parts