However, "room to let" means "room to rent", which seems quite different. However, the origin of the word shows us that it may not be so strange. If you are a landlord, you rent a house and you want a tenant to rent it. To rent is a verb that means to allow someone to make use of the property or to live in it temporarily in exchange for a payment.
This payment is monthly and is also called renting the place. To rent means to be available for rent, but when a property has been rented out, it simply means that it is no longer available for rent.