Amplification.

An Aerial sat on your roof  produces a voltage that  is fed into your television.
This voltage varies with the necessary information to produce a TV picture.
The strength of this voltage supplied by your aerial can be very high, or low depending on what your view to the TV transmitter is, if you have line of sight,then  you can split the signal into several rooms without the voltage dropping or your picture breaking up or freezing.

But if you live in a valley or local dip,then you are starting to look through solid earth which knocks the voltage level down.
If this is too severe then you will need to increase the signal using an amplifier,very much the same as turning up the volume on your HI FI

Below is a mast head TV aerial amplifier.
This attaches, unsurprisingly  to the TV aerial mast,amplifying the signal to allow for losses in the co ax cable,which are pretty large.
A  power unit  powers  the amplifier through the co axial cable,through a process known as diplexing,thus getting around the need for a mains point,a quick powerful way of overcoming poor signal reception.
Both these units are referred to as a TV  Aerial booster.