What do you mean by Income from House Property?

Unlike the other heads of income, Income from house property is a notional income based on a concept called Annual value. This is the value a property is expected to fetch if it is let out. It may be more than the actual rent being received if let out. If it is not let out the expected market/fair rent will be considered as annual value for the purpose of taxation. Property includes the building and the land surrounding it.

Could you please explain to me what "let out" means here?
To Let/ Let out means to let somebody rent it (property).

Substitute 'Let out' with 'Rented out' and it should make sense.
let out = rented out

to rent out = to allow people to live there in exchange for a periodic payment to you, the owner