Speed describes a rate because it quantifies the change in position of an object over time. It expresses how fast an object is moving or how quickly its position changes. Speed is calculated by dividing the distance travelled by the time taken to cover that distance.

Mathematically, speed is represented as:

```

Speed = Distance / Time

```

The units of speed can be kilometres per hour (km/h), metres per second (m/s), miles per hour (mph), or any other combination of distance and time units.

Speed provides information about the rate at which an object moves, but it does not specify the direction of motion. For that, we use velocity, which is a vector quantity that includes both speed and direction.