It usually means, "I am ready to enjoy..."

Student or Learner
When a person says, "I am down for this movie" what exactly does it mean?
It usually means, "I am ready to enjoy..."
It means they agree to go, that they're in the mood to do it.
"If you want to go to a movie I'm down with that, as long as it's not one of those stupid chick flicks."
[native speaker, not a teacher]
It is more usually "down with." I think "down for" is just a variation.
We put our names down for things in the US too. But when it's your turn to do something, we say, "You're up, John."
"His number was up" is also said about death:
"Twelve hours after his dramatic rescue from the burning wreckage of a plane crash, Fred Smith was killed while crossing the street to mail a letter."
"Wow. I guess his number was up."
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