Carrier pre-select is a regulator imposed system that compels telecoms networks to allow customers to route all their calls through another (a competitor's) network.
The importance of carrier pre-select is that it allows competitors to use an incumbent's infrastructure.
The call is carried by the telecoms company that controls the local loop as far as its exchange, but is rerouted to the customer's chosen carrier there.
Although this is superficially similar to local loop unbundling, it is far more limited in its effects. As it does not require the installation of another network's equipment in telephone exchanges (but merely a connection from each exchange to the other network) it is far cheaper and simpler to roll-out. This also means that the incumbent teleco retains line-rental revenues and control over the provision of the line. It also usually only applies to voice, not data.