Escrow in general is just a service that acts as a middle man.
1.) Holds Payment from the Buyer
2.) Verifies domain has been transferred
3.) Releases Payment to the Seller
Bido Escrow is basically referring to the name being held in their account, so they can take care of the transaction after the auction is over. Also, since it is in their account the seller can't back out if the auction doesn't go as well as expected.
Brad