Escrow is an arrangement where a neutral third party holds funds or documents until the agreed conditions are satisfied, then releases them. In real estate it shows up in two common places: during a purchase, an escrow agent holds the buyer’s earnest money and paperwork until closing; and with a mortgage, the lender may keep an escrow account to pay property taxes and insurance out of your monthly payment.
The point is trust — neither side has to rely on the other’s word, because a third party controls the money until the terms are met. Some jurisdictions also require security deposits to be held in a separate account, a related idea of keeping funds segregated.