On an automatic transmission gear selector, “D” stands for Drive.
Selecting Drive tells the transmission to move the vehicle forward and use its available forward gear ratios. In a conventional automatic, hydraulic or electronic controls choose when to shift based on factors such as vehicle speed, engine load and throttle position. The driver normally controls acceleration with the throttle and stopping with the brake, without operating a clutch pedal.
Most automatic selectors follow a pattern such as P–R–N–D–L: Park, Reverse, Neutral, Drive and Low. The exact layout can vary, and modern cars may add labels such as S for Sport, M for manual selection or B for increased regenerative braking in some hybrids and electric vehicles.
A common mix-up is assuming D means one fixed gear. In most automatics, Drive permits the transmission to shift through its full forward range; it is not simply “fourth gear” or another single ratio.