Here's a breakdown:

* Velocity: Velocity is a vector quantity that describes both the speed and direction of an object's motion.

* Rate of change: This means how much the velocity changes in a given amount of time.

* Time: The time interval over which the velocity changes.

Formula:

Acceleration (a) is calculated using the following formula:

```

a = (v_f - v_i) / t

```

Where:

* a: Acceleration

* v_f: Final velocity

* v_i: Initial velocity

* t: Time

Units:

The standard unit for acceleration is meters per second squared (m/s²).

Key Points:

* Acceleration is a vector quantity, meaning it has both magnitude (amount) and direction.

* A positive acceleration indicates that the object is speeding up, while a negative acceleration indicates that it is slowing down (also known as deceleration).

* Zero acceleration means the object's velocity is constant (not changing).

Examples:

* A car speeding up from rest (initial velocity = 0) is accelerating.

* A car braking to a stop is decelerating (negative acceleration).

* A car moving at a constant speed in a straight line has zero acceleration.