C'est si bon means “It's so good” or “It's so nice.” It can be used to talk about just about anything, from food, to an activity. You may know this phrase from the title of a famous jazz song that's been sung by artists like Dean Martin, Eartha Kitt, and Barbara Streisand.

Or if you've just enjoyed a nice holiday and somebody asks you how it was, you might say 'yeah it was good', i.e. 'oui c'était bien' (c'est bien in the imperfect). 'C'est bon' on the other hand can be roughly translated as 'it's fine' or 'that's ok'.