Literally the dogs testicles. In British slang, bollocks, means "testicles", however, when it is applied in this context its usual and derivative meaning is "the best", or "the greatest", or "superb"
interchangeable with the expression "the mutts nuts
usually is applied in a similar fashion to: "That jacket is the dogs bollocks as far as I'm concerned"
or "when are you having the dogs bollocks lopped off"?
A Television chef might stir the pan, taste the contents, and then look into camera while saying,
"You know, these Korean meatballs
REALLY ARE the dogs bollocks"