L1, L2, and L3 are the input three-phase terminals in a contactor.
T1, T2, and T3 are the output three-phase terminals.
L1 is switched to T1, L2 is switched to T2, and L3 is switched to T3 when the contactor is turned ON.
The “L” in L1, L2, and L3 stands for “Line”, meaning the incoming supply line.
The “T” in T1, T2, and T3 stands for “Terminus”, meaning the exit point, i.e. going to the last thing connected in the circuit.