Introduction

Think of it this way... Why does an egg crack when it hits the floor but not if it hits a pillow on the floor?
The reason is that the pillow causes the egg to slow down which increases the time it takes to stop and therefore creates more give for the egg.

Momentum is the mass times velocity of an object.
An impulse is a force over a period of time.
An impulse is a change in momentum.

Another example is a car crash it doesn't matter if the car crashes into a wall or if it hits another car head on.