when you jump, your legs apply a force to the ground, and the ground applies and equal and opposite reaction force that propels you into the air.

Forces, the way Newton described them, change motion.

So when you throw a tennis ball against a wall, the tennis ball obviously applies a force to the wall.

What's less obvious is that the wall applied a force to the tennis ball. Did the motion of the ball change? Yes it did. What caused it? The wall. So, using Newton's terms, the wall applied a force to the ball.

Turns out, the wall applies the exact same amount of force back as the ball applied forward.

*EDIT: The comments below are excellent expansions. The wall doesn't seem to move, but there are vibrations throughout the wall (and so on) that account for that force. And yes, many other forces are acting on the wall.

EXPLAIN IT TO ME LIKE I'M 10: Maybe it's better to say "forces affect inertia - they can help or hurt it."

If your question is about how anything can move at all "by Newton's third law", please see https://physics.stackexchange.com/questions/45653/given-newtons-third-law-why-are-things-capable-of-moving

"For every action, there's an equal and opposite reaction"

Basically, what it's saying is forces always comes in pairs. For every force, there's an equal force in the opposite direction.

The force pushing air out of a balloon has an equal and opposite force that pushes the balloon forward.

The force from a boat's propeller pushing water backwards has an equal and opposite force that pushes the boat forwards.

The force that shoots a bullet out of a gun has an equal and opposite force that causes the gun to recoil.

Did the motion of the wall change? Yes it did.

Being rigidly attached to the 6.6 sextillion tons of planet earth does tend to diminish the magnitude of the change in motion though.

It flexes at the point of impact.

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Given the density of a standard wall (assumed concretish) vs a tennis ball, surely you'd be going smaller than a micrometer to measure flex.

Our walls here are usually drywall with supporting wooden studs. The flexure is invisible but easily measured

When object A acts on object B, object B also acts on object A, but on the opposite way.

For example, picture a cat sitting on the floor.

The earth is pulling the cat down through gravity, but the cat is also pulling the earth up through its gravity.

These forces are opposite but equal to each other.