Each week, #mochat has a dedicated topic, but as with anything on Twitter, the conversation might get steered in different directions based on the community. Chats are generally moderated by @conigs, @caitcadieux, and @flabbyironman.
The easiest way to participate is to log in to tchat.io before the chat (Tuesdays 9PM ET). The page automatically refreshes to show the most recent posts and adds the #mochat hashtag to your posts. You can also use any Twitter program (mobile or desktop), just search for the hashtag #mochat.