When you're up for it, you feel like doing it or having it. "It" can be any activity or thing: going to dinner, a movie, bar hopping, camping, whatever. When you want to do something, say swimming in the river, and you'd like your friend to join you, you can ask, "Are you up for it?" Your friend can then say, "Sure. I'm up for it. Let's go."
You can substitute "it" for its antecedent. So you can say, "Are you up for a movie tonight?" or "I don't think I'll ever be up for bungee jumping." or "I'm up for some pizza. What about you?" This is a common expression that can be used in just about any situation.
A: Are you up for a game of chess?
B: I've never learned how to play chess.
A: Common, I'll teach you. It's easy. Besides, it'll help kill time while we wait for Ben.
B: Alright, let's do it. I think I'm up for learning a new game. How long is Ben's exam, anyway?
A: It should only take about an hour, but he may take a little longer. He said he hadn't studied.
B: I thought he had time to study last night.
A: He did, but he just wasn't up for it. I think he's just tired of school.
B: Well, I hope he does well.
say - for example
antecedent - the word or phrase that a pronoun refers to
bungee jumping - an adventure sport where you jump off a high point with a stretchable rope tied around your ankles
just about - almost
kill time - let time pass (see the June 15 blog)
Alright folks, I hope you're up for practicing English today. Just speak as much as you can.