In tennis when you serve the tennis ball the momentum of the ball isn't much because you're tossing the tennis ball up and it doesn't much velocity because of how high you have to toss it.
But when the tennis ball hits the racket the impulse of the momentum increases significantly because the force that the racket gained in the swing increased the velocity of the tennis ball which increased the momentum causing it to fly towards your opponent and the goal of tennis is to try and make sure that your opponent can't hit return your hits and score as much as possible.
And when your opponent goes to hit the tennis ball back to you they feel the force and momentum of the tennis ball that you applied to the tennis ball.
And the impulse changes because that momentum was changed by the tennis racket but the tennis player sends it back by applying more force to the tennis ball causing it to be sent back to you with greater or equal force and momentum.
When you hit the ball you want to increase its momentum so you want to change its impulse by increasing its velocity and the more force you gather and the longer the ball getting accelerated by the force of the swing through the racket the faster and harder it becomes to hit the tennis ball.
So to win a match in tennis you want to have the tennis ball gain as much velocity as it can to increase its momentum and to do that is to put a good amount of force into the swing of the tennis ball while giving the ball enough time to accelerate through the swing.