Escrow is when a "middle man" (a reputable, trusted third party) is used to conduct a transaction between a buyer and seller.  In your case, since you are looking to buy BTC, after reaching an agreement with a seller, the seller would send the BTC to the escrow agent.  The escrow agent would notify you that he has recieved the BTC and you would then send your payment to the escrow agent.  Once the escrow agent has recieved your payment, it would be forwarded to the seller and the BTC would be sent to you.