You may have encountered this term before, abbreviated as LLU or LLUB. It refers to a pretty straightforward trend in the broadband industry where the services run on BT lines are opened up for use by other companies. LLU developments have occurred in most modern countries, and in most cases are opposed by the incumbent company who used to have control of the network, BT in the case of the UK and AT&T in the US, but it is argued that the networks were built in an noncompetitive time and therefore should be accessible by all customers. The movement for unbundling began in the late 1980s for phone services.
The local loop just refers to the network, and unbundling means opening it up to other companies. Unbundling means that other providers can offer their own services and setup with on the network, and not just repackage BT services. The new provider implements their own technical setup and software but relies on the physical BT network.
In the age of broadband this also refers to the ability to get phone services from one company but broadband with another. You also have to pay BT for line rental, while the provider also pays BT for use of the network. The whole unbundling trend has allowed for a more modern telecommunications market, with more competition, variation and overall cheaper offerings for the consumer. This has kept BT on their game and delivered a lot of benefits for broadband customers.
And for many customers there are yet more options available - aside from mobile broadband, there are fibre and cable offerings that either set up a new line or go through cable connections that were previously used for TV only. So thanks to these developments and local loop unbundling there has been a great improvement in the options for broadband users.