Yes, a trustworthy person for both the parties of a transaction.
There are some deals in which the 2 parties won't be mutually agreeing on who will send the money first, they can hire an escrow for that. He/she will take from one party and send it to the other.
It's not as easy as it looks. Lots of complications will be there with many of the transactions. And he'll be a decisive person to complete the deal.