If something is up for grabs, anybody who is interested in it can try to take it. This can be anything. For example, if there are leftover cookies after a party, they are usually left on the table and are up for grabs, which means anyone who wants one can have one. In another example, if a job opens up in your department, it's usually posted for everyone to see, which means it's up for grabs for anyone who is qualified and who wants it.
A: I hear the lottery jackpot now is ten million dollars.
B: So no one has gotten the winning ticket yet?
A: Nope. It's still up for grabs.
B: I think I'll stop by the store and buy one. You never know.
B: So did your son make the cut on the school baseball team?
A: I don't know yet. I'll find out when I get home. There were still two spots that were up for grabs yesterday, so he was keeping his fingers crossed.
B: Well, I hope he gets it.
opens up - becomes available
make the cut - be selected (for a team or some kind of group)
keeping his fingers crossed - hoping for the best (see July 20 blog)
Alright folks, the next time you bring some candy or chocolate to school, and you want to share it with your classmates, you can announce, "This is up for grabs, everyone. Help yourselves!"