We have to remember the definition of impulse, which is a force that acts on a particle for a short interval that is large enough to produce a definite change in the amount of motion is known as impulsive force and the resulting movement is called impulsive movement.
Therefore impulse is equal to the product of force by the time interval.
The graph shows the axis of force and the axis of time, therefore the area under the curve will define the value of the impulse for that body.
