Slang version of the saying "That's the main thing".
Originating in the South Wales valleys and mainly associated with 'Chavs', it often has no real meaning or connection to its predecessor when used in conversation.
Girl: "Oh my God, I totally nearly just died"
Boy: "Issit? Thasa main innit"
Boy 1: "We're going out tonight!"
Boy 2: "Issit? It is issit? Thasa main innit!