A Home Location Register (HLR) is the definitive database of mobile subscriber information for a wireless carrier's network. It is the real-time list that matches phones, phone numbers, user accounts and service plan information. It also keeps track of the user's location on the carrier's network, (or another carrier's network if roaming,) so incoming calls can be routed through the correct tower.
HLRs use the ANSI-41 or MAP protocol to identify and authenticate users, and manage roaming situations involving other networks.
The HLR is used to control what kinds of calls each user is allowed to make and how they are handled. The HLR can also control access to other services such as data.
The HLR also tracks individual devices, and can be used to blacklist phones that have been reported stolen.
The HLR is key to any wireless carrier's network, so an outage or problem with the HLR typically causes an effective outage or other major problems for a mobile network.
Larger carriers often have multiple HLRs, in which case each subscriber "belongs" to only one HLR.