Coaxial cable have an inductance and capacitance and for TV is rated at 75 ohm. To connect two cables into one you need some passive components to match to two cables into one. If you get a Y splitter this contains these passive components.
To work correctly both cables also need terminating into a 75 ohm appliance leaving one disconnected will effect both signals plus of course both TV's only get half the signal if the split is perfect.
However if you use an active splitter the electronics sort this all out and also boost the signal so both TV's get full signal.
For an active splitter to work it must have power there are three main ways to power these units. Batteries, Mains connection, power through coaxial cable.
For the latter either the receiver as with Sky boxes or a power supply sends power up the same cable as the signal comes down which means you don't need to have power in the loft.
Most the active splitters which take there power from the coaxial cable are referred to as mast head amplifiers. These can still be used in the loft etc.
Because of the sending of signals in two directions and joining FM, DAB, TV, Sky signals together and then splitting them there are a whole host of units some will pass through power others will not and some will even send the remote control signals as well.
The price varies from around £10 to £100+ according to what you want it to do.
I would think you need one of
these on mast head and also one of
these to power it which will cost around £22 don't forget you need both bits. The unit with plug on is not an amplifier but is a power supply which sends power to amplifier on the mast. The closer the amplifier is to aerial the better they work.