Summary
FTTC means fibre to the cabinet. It can be useful where full fibre is not available, but the copper section from the cabinet to the home means speed depends heavily on distance and line quality.
Definition
FTTC, or fibre to the cabinet, uses fibre to reach a street cabinet and then uses a copper phone line for the final connection into the home. The longer and poorer quality that copper section is, the more speed and reliability can fall. This is why two homes in the same postcode can see different estimated speeds. For a buyer or renter, the key check is not just whether FTTC exists, but the estimated speed range and the minimum guaranteed speed at the exact address.