Absolute threshold simply means what someone can detect and see when it is closer to the person.
A person can detect a single drop of perfume diffused in an area the size of a one-bedroom apartment because the apartment is closer to the people. In a case where the apartment is far away from the person, the person will not be able to detect the single drop of perfume. EXAMPLE if an aeroplane is flying in the sky someone may not be able to detect and see how big the aeroplane is because it's far away from the person except if the aeroplane lands that is when the person will see how big it is because it's now closer to the person.
