Loop-in wiring is the method where the phase and neutral run from the distribution board to the first switchboard, then loop on from that switchboard to the next, and so on, with every joint made at a terminal inside a switchboard or ceiling rose rather than in a hidden connector somewhere in the conduit. Each light or fan is fed by a short branch from the nearest switchboard. The alternative, joint-box wiring, makes the connections in separate junction boxes, which is more wire and more places for a joint to loosen unseen.
Electricians prefer loop-in because every connection is behind a removable plate, so a fault can be traced and a circuit extended without opening walls. It also uses less cable. The one discipline it needs is a conductor that can be looped repeatedly without work-hardening or breaking, which is why flexible stranded house wire such as Shakti FR LSH is used rather than solid conductor. Neutral must be looped as carefully as phase; a borrowed neutral between circuits is the commonest loop-in mistake.