D-Pad is stood for Directional Pad. It is used by the gamer to give instruction for the games they play. D-Pad is used by Nintendo for the first time for their Famicom console. The same controller was later introduced for the Nintendo Entertainment System (NES).
Before the D-Pad, there are many types of a game controller which also capable of moving a character in a game. But the D-Pad design is comfortable and easy to use. Compared to other controller design, the D-Pad allow gamers to slide up, left or right without having to lift up their left thumb.
In the following video, Gaming Historian walks you through a history of the D-Pad and he also shows other game controllers back in the good old days.