I like these questions because even as a native speaker they make me think about things I take for granted :) "And so forth" is often coupled with "and so on": It essentially means "and more of the same". Imagine an example where you're showing a new employee around your office, you might say something: "and here we have the print room, with all the printers and paper and stationary and so forth" - meaning: "...and all the other printing relate stuff you might need" Hope that helps.

I you are writing text or a sentence it means with additional related or similar things mentioned (but not specified). For example, "when I come to the party I will bring cake, party hats, games, and so forth."