What is an antenna?
An antenna is a transducer, i.e. a "converter" between two media. The antenna converts electromagnetic energy in a cable into electromagnetic radiated power in free space.
Various characteristics of the antenna determine "how efficient" this conversion is performed. To have as much energy as possible carried on as radiation in the room is of great importance, i.e. the antenna should not be capable of consuming energy itself.
It is important that the antenna concentrates its radiated energy as efficiently as possible in the required way. If so, either a wide coverage range or a reduced transmitting power can be achieved, until the range is exactly suiting the one required.