From :help tag-stack:
On the tag stack is remembered which tags you jumped to, and from where.
Tags are only pushed onto the stack when the 'tagstack' option is set.
The output of ":tags" looks like this:
# TO tag FROM line in file/text
1 1 main 1 harddisk2:text/vim/test
> 2 2 FuncA 58 i = FuncA(10);
3 1 FuncC ...