From domain driven design perspective DbContext is the implementation of UnitOfWork and a DbSet<T> is the implementation of a repository.
This is the point where DDD and EntityFramework contrast. DDD suggests to have a Repository per aggregate root but EntityFramework creates one per Entity.
So, what is an aggregate root?
Assume that we have a social network and have entities like Post, Like, Comment, Tag. (I believe you can imagine the relations between these entities) Some of the entities are “Aggregate Root”
To find the aggregate root(s) I try to find which entities cannot live without the other. For instance, Like or Comment cannot live without a Post. Then Post is an aggregate root and we need a PostRepository or turn the Post entity into a Repository (the famous collection like interface thing). CRUD operations for Comment and Like (as well as the Post) should remain on this repository.