When you are "up for" an activity, it means that you are willing to do it.
When you ask a friend "Are you up for ___?", you want to know if they want to do it, and whether they have enough energy for it. For example, if someone asks:
You want to play a game of basketball?
...if you're too tired right now, you could say:
I don't think I'm up for it right now.