Escrow is a specific type of contract, where a third party entity receives documents, funds etc ... from one transaction party and only releases them, when the other transaction party meets the specified requirements.
Escrow services are often used in case of selling digital assets. For example when selling a domain name, the buyer would deposit the agreed sum to an escrow service company, which would release them to the seller, once the ownership transfer has been confirmed.
Escrow contracts can also be executed by using smart contracts on a blockchain, for example in case of Atomic Swap transactions.