It's the same as "the bee's knees".

This phrase is very outdated. You will not hear it at all now. If something is "the cat's pajamas", it is very good, very positive. It usually referred to a person. It was in use in the 1920's. Sometimes you will hear it to deliberately tag something as old. There is a company called "The Cat's Pajamas" which specializes in vintage (old-fashioned) clothes.