Acceleration

Acceleration is the change in velocity divided by the time:

$$a = \frac{v - v_0}{\Delta t}$$

Unit: $\mathrm{m/s^2}$. $2\ \mathrm{m/s^2}$ adds $2\ \mathrm{m/s}$ every second.

Each second the velocity grows by $2\ \mathrm{m/s}$, so the car covers more road each second.

A car braking from $20\ \mathrm{m/s}$ to $8\ \mathrm{m/s}$ in $4\ \mathrm{s}$ has $a = \frac{8 - 20}{4} = -3\ \mathrm{m/s^2}$. Velocity and acceleration pointing opposite ways: the object slows down.