Well in simple terms; Chess that is slow.
Slow chess

I assume you mean Daily and not Live. If so,
https://support.chess.com/customer/en/portal/articles/1444775-how-does-turn-based-chess-work-

I don't know about that Martin. I'm assuming he meant live since the OP is a member of this group:
https://www.chess.com/groups/view/slow-chess-tournament-3015
Anyway back to the question. It's just another term for standard chess as opposed to blitz or bullet. Of course one man's slow chess is another's rapid...
What is slow chess, I've just joined?