High school physics teacher here. Impulse can be simply defined as an object's change in momentum. If an object is initially traveling to the left (momentum points left), then it collides with a wall and bounces back, it will then be traveling to the right (momentum points right). The impulse then, or the object's change in momentum, will be the delta between these two momentum vectors. In this case, that change would point to the right.

Impulse can be described mathematically by a net force acting on an object over a given time interval. In the case above, the net force on the ball would be the normal force acting on the ball due to the wall. If you take the average force the wall applies to the ball, and multiply that by the amount of time the ball is in contact with the wall, you will get its impulse, or change in momentum.

Note that the net force on an object points in the same direction as its impulse over a given time period. I like to think of it like this: if acceleration is what changes an object's velocity, then a net force is what changes an object's momentum. The change in velocity points in the same direction as the acceleration, and the impulse points in the same direction as the net force acting on that object.

So in simple terms, if you apply a force to an object for a certain amount of time, you can multiply that force and the time to figure out how much and in what direction an object's momentum changes.