Mark as read button?

[Dodge41]

Fairly new to chat, so please advise how to work the "Mark as read button", my geriatric brain can't find the answer.
Thanks Ausfish and also for he great site one of the best particularly being Qld based

[imported_admin]

It marks all the topics in theta catergory as read. This way NEW will not show up next to the threads. Then you can see what NEW posts have been posted from that time on.