This tag is for questions and bug reports related to the SE chat system.
The SE chat system allows real-time discussions between users, and can be found here. The official Mathoverflow chat can be found here.
You can find out more about chat in faq.
If you want to use TeX-syntax for discussing mathematics, have a look here to see how to get it rendered in chat. (There is also a related post on this meta: MathJax on Chat.)