Speed is a scalar quantity that refers to "how fast an object is moving" Speed also can be thought of as the rate at which an object covers distance. A fast-moving object has a high speed and covers a relatively large distance in a short amount of time. Contrast this to a slow-moving object that has a low speed; it covers a relatively small amount of distance in the same amount of time. An object with no movement at all has zero speed. Everything that moves has a speed.
There are many measurements of speed. (km/h,m/h,) etc.
Speed is mainly used by vehicles. Without speed, we would not know how fast we are driving/riding etc. Furthermore, there also wouldn't be speed limits. Hence, vehicles would try to speed and there will be a high chance that an accident would happen.