Yes they are free to use though subscription channels are also available as extras if it has a slot for a smart card.
To use it follow these instructions:
Plug the digital terrestrial receiver into the mains.
The aerial plugs into the digital terrestrial receiver.
The digital terrestrial receiver connects to your TV via a Scart lead.
Switch the TV to AV mode. If it has more than 1 you may need to try both AV1 and AV2.
Use the remote control for the digital terrestrial receiver and select 'menu'. You should then get the option to tune it in or search for channels. Leave it for a couple of minutes while it does so and Bob's your uncle.