With 1-phase rails, the electricity runs between two wires, namely, the phase and null wires. This means that all light sources on the rail are operated at the same time and thus are always switched on or off together.
3-phase rails use 3-phase wires that are combined separately with the null wire. This makes it possible to have three different circuits on the rail (e.g. spots, TIs, and pendant lamps). All three can be operated separately.