LLU or Local Loop Unbundling is term relating to ADSL internet connections, which in the UK are mostly owned by Openreach.
ISPs can sell you their version of broadband by using BT’s infrastructure.
A local loop is the cable through which you receive internet and phone calls.
Unbundling is what allows different ISPs to connect you to their service.
Unbundling typically refers to internet connections as it is possible to retain a BT phone line while receiving broadband from another ISP.