Certain households thoughout the country have little boxes attached to their tv sets, these monitor what they watch and this is then multiplied up to reflect that percentage of the population. I am sure someone will be able to give you a more percise definition but that is the basics as I understand it.
Warpig3 has provided an accurate answer. The figures are collated by the Broadcasters' Audience Research Board Ltd (BARB). Their methodology is explained on their home page, here: http://www.barb.co.uk/