It is probably just to describe a wide, open smile. But the exact meaning would depend on the context (who is smiling, where, why, in response to what, etc.). It might be simply dark, and he has very white teeth! Or it might be to underline an aspect of character: openness, honesty, naïvety, simplicity etc. or, possibly, menace. Basically we need more context to fully judge the meaning here...

Thank you. I got it and appreciate your useful answer