Don't worry a bit! It's a strange little thing, but is the absolute dream event to an author! A pre-empt is one of two things that are similar, but not quite the same as each other. When a book/screenplay has been sent to a variety of publishers/studios and there is more than one offer, the agent will start the steps to hold an auction for the rights. Now, two things can happen:
1. One studio/publisher is REALLLLLLY hot for the property and bids an obscene amount of money to prevent the auction altogether. The agent knows there's no chance that anyone will outbid the house, so there's no auction and the studio/publisher acquires it in a "pre-empt."
2. The auction begins. The agent calls around for the first round of bids. Then s/he calls around and says "Okay, here's the highest/best offer. Who wants to bid higher?" Several rounds can occur before one bidder decides "I'm sick to death of this. Let's just end it." They take the last bid and double or triple it. The others say, "Yikes! Too rich for my blood!" and bow out. This is ALSO a pre-empt because, again, it stops the auction. But there's more knowledge involved in this (as in, the bidder knows for certain that others were interested, so they'll probably crow about it a bit more.
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Does that help?