It causes a change in the object's momentum (impulse).

The total momentum of the system before and after the collision remains constant.

External forces cause a change in the system's total momentum.

Momentum is a measure of how much 'oomph' an object has when it's moving; its resistance to changes in motion.

$p = mv$, where $p$ = momentum, $m$ = mass, and $v$ = velocity.

Impulse is the change in an object's momentum caused by a force acting over a period of time.

$J = \Delta p = F\Delta t$, where $J$ = impulse, $\Delta p$ = change in momentum, $F$ = force, and $\Delta t$ = time interval.

A closed system is one where no matter or energy is exchanged with the surroundings; total momentum remains constant.

An open system is one where matter or energy can be exchanged with the surroundings; total momentum may not be constant.

The area under the curve represents the impulse.