Theory:

Consider that you're stooped at the signal due to traffic, and once you see the green signal suddenly you're increasing the speed of your bike to move fastly, and this term is called acceleration.

Acceleration (a):
The acceleration can be defined as the rate of change in velocity.

When the object undergoes acceleration its speed and/or direction change(s).

If an object changes its speed or direction then it is said to be accelerated.

$\mathit{Acceleration}\phantom{\rule{0.147em}{0ex}}\left(a\right)=\frac{\mathit{Change}\phantom{\rule{0.147em}{0ex}}\mathit{in}\phantom{\rule{0.147em}{0ex}}\mathit{velocity}}{\mathit{Time}\phantom{\rule{0.147em}{0ex}}\left(t\right)}=\frac{v-u}{t}$
The change in velocity can be written as $\mathit{Final}\phantom{\rule{0.147em}{0ex}}\mathit{velocity}\phantom{\rule{0.147em}{0ex}}\left(v\right)\phantom{\rule{0.147em}{0ex}}-\mathit{Initial}\phantom{\rule{0.147em}{0ex}}\mathit{velocity}\phantom{\rule{0.147em}{0ex}}\left(u\right)$.

Where $$v$$ is the final velocity of an object and $$u$$ is the initial velocity of an object.

The SI unit of acceleration is $m{s}^{-2}$.

Types of acceleration:
1. Positive acceleration.
2. Negative acceleration.

Positive acceleration:
If the velocity of an object increases with respect to time, then the object is said to be in positive acceleration or just acceleration.
Example:
A train moving with increasing speed has positive acceleration.
After the green signal the vehicle moving fastly.
Negative acceleration:
If the velocity of an object decreases with respect to time, then the object is said to be in negative acceleration or deceleration or retardation.
Example:
A running bike slowly stops.
A rolling ball that slowly stops.