Very simply, it is a deposit of
funds, a deed, or other instrument by one party for the delivery to
another party upon completion of a certain condition or event
agreed to by all parties to the transaction
The escrow "holder" must
 safeguard the
funds and/or documents,  disburse the funds, and/or  convey title only when all provisions of the
escrow have been fulfilled. Usually, this function is performed by an attorney.