HLR stands for Home Location Register which is part of the telecommunication infrastructure. It is essentially a central global database of all the cellphone subscribers registered with a particular mobile network and continually updated in real time. HLRs hold the most accurate, up-to-date information on any cell phone subscriber anywhere in the world.
By making a query to the Home Location Register, the lookup service determines whether that number is valid, whether it is currently active in a mobile network, and if so which network, whether it was ported from another network, and whether it is roaming. The query will also return meta information such as the IMSI (International Mobile Subscriber Identity), MSC (Mobile Switching Center), MCC (Mobile Country Code) and MNC (Mobile Network Code).