In a brand new Stacks 3 installation the Templates library group is empty by default. So, the first thing to know is that this is OK. Everything is working fine.
The more detailed answer is that there are some things that can go into the Templates library group. Some 3rd party developers sell and/or include templates with their stacks. And Stacks 2 used to come with some installed.
If you install template stacks from a 3rd party they will show up here.