Market rent is the amount a comparable unit would rent for right now given current demand in its area — as opposed to the “contract rent” a sitting tenant is actually paying, which may lag the market. The gap between the two is often called a “loss to lease.”
Landlords estimate market rent from recent leases and active listings of similar units nearby, adjusting for size, condition, and amenities. It informs how you price a vacancy and set a renewal increase, though where rent regulation applies, what you can actually charge may be capped below the market figure.