When something is cracking it is particularly good or excellent. People can also be described as “cracking.” For example: “He’s a cracking lad.” It can also mean to get started on something (and this meaning is also used in American speech).
Another variation is the word “cracker.” Describing a person as a “cracker” means you think they are fantastic. (But watch out! This is completely different in American slang. In the U.S., “cracker” can be an insulting term for white people from rural areas.)
“That was a cracking dinner. Compliments to the chef.”
“I have lots of Christmas presents to wrap. I’d better get cracking!”