Stickler

Could anyone explain what does stickler mean.
Thanks

One meaning is a person who is very precise, particular about a subject. "He is a stickler for good manners."

From Merriam-Webster online:

One entry found for stickler.
Stickler
Main Entry: stick·ler
Pronunciation: 'sti-k(&-)l&r
Function: noun
1 : one who insists on exactness or completeness in the observance of something. A stickler for the rules.
2 : something that baffles or puzzles :