Acceleration is a vector which describes the rate of change
of velocity of an object.
The average acceleration over some interval is the total change
in velocity during that interval, divided by the time.
The instantaneous acceleration at some moment in time is the
change in velocity of the object right now!
Instantaneous acceleration is the derivative of velocity with respect
to time.
When an object moves with constant acceleration, there are
a set of 4 equations which relate its position, velocity,
acceleration, and time. You will need to memorize these
equations to solve many problems.
v - v0 = a * (t - t0)
x - x0 = 0.5 * (v + v0) * (t - t0)
x - x0 = v0 * (t - t0) + 0.5 * a * (t - t0)^2
v^2 - v0^2 = 2 * a * (x - x0)
Why do we put such an emphasis on constant acceleration?
because it does occur frequently in the real world
because the kinematic equations for constant acceleration are
pretty easy to use