You may mean "crackers", which means crazy. Or you may mean, "that's a cracker", which means "that's a great thing." It's a general compliment e.g. That was a cracker of a game between England and Wales last night. If you use he or she, a "cracker" can also mean an attractive person. These are slang expressions common in the UK but I don't know how common they are elsewhere.

How was it used?