marissa gave you the short answer, but I'll give some more background info. It's the abreviation of 그리고 나서, which is an incorrect form of 그러고 나서. So the correct form is 그러고서, not 그리고서. It literally translates to "after doing so", thus in short it means "then". 그리고 is a separate word and means "and". But following context it could also mean "then". "And" can mean "then" in English too, so don't worry about it.

Then, next