A multisignature escrow is a special Bitcoin Cash address in which several people must sign a transaction in order to move the funds. In Bitgree 2-3 multisignature is used, so 2 signatures of 3 people are required. The people who can sign are wish's user, proposal's user and Bitgree.
Under normal conditions, wish's user signature plus proposal's user signature are sufficient to move the escrow funds. But in the event of a dispute between them Bitgree comes into play, which will be decisive in moving the escrow funds, thus unlocking the situation.