A “hold” means an employer has the right to book you first, before you take a job anywhere else. if you get “challenged” by another company to book you for the same time, you have to go to the company who is “holding” you and ask them to make a decision.
They have 24 hours to decide if they want to turn their “hold” into an official “booking” (which basically means they are obligated to hire/pay you). If they can’t book you, they’ll “release” you and you’ll become “booked” at the other company.
None of this, of course, has any legal obligations anywhere but there is some serious political correctness and responsibility you need to have when being a freelancer. Of course, studios should give you the same amount of clarity and respect.
My advice: have “holds” at multiple places, because these things fall through rather quickly.
As David points out, holds aren’t regulated by law or contract. But they are an established part of doing business in major markets, and it’s a really bad move to break these unwritten rules. Honor the system and it will honor you.
Thanks to Fred Kim for hooking this up!