What is single phase dc system?

Single-phase means you have a simple AC supply on two wires, usually a live and neutral, as supplied to the average house throughout the world. Single-phase also includes a split-phase system where you have for example a 240 v supply with a centre-tap neutral giving two separate 120 v supplies.

The alternative is three-phase, which can be on three wires or it can include a 4th neutral wire. It is used for transmission of power in bulk because it uses wire efficiently over long distances.