Escrow stands a middleman in a certain transaction between two parties. Since they are so called middleman when both parties are dealing with a transaction, they must be trusted.

For a much simply explanation, here is some basic example: Person A selling a product , Person B wants the product. Since no such thing as instant exchange they need to use escrow services to make the transaction deal quite safe. Person B will give the money to the escrow and after that Person A will give the item to buyer. After Person B confirms that all are smooth then the escrow can now give the amount that Person B handed to him.