Literally, it means "in order." It generally means "ok" or "fine" or "everything will be ok." You can even end your sentences with it, like tacking "right?" to the end of the sentence.
But really, it seems to mean anything you want it to mean, so long as you are answering with an affirmative response of some kind. However, I've never heard it used enthusiastically. It reminds me of how Americans say "fine." Alternatively, when I'm stressed, I can stop and say, "B'seder." It will be ok. Soon, Gdwilling.
Some questions you can answer with b'seder:
- How are you?
- How about getting dinner next week?
- Is everything b'seder?
- Will you marry me?
- How was your final exam?
- How was the party?
My favorite quote about b'seder is "It does seem a little ironic to me – if my translation is right, 'b’seder' actually means 'in order,' but it seems to be used when nothing is in order at all."