Basically, any energy has to go somewhere. If you hit a billiard ball into another, the second ball will travel away from the first, and the total energy after the hit will be about the same as before (minus friction and other losses). Pushing someone forward will tend to push you backward, etc.

If you thinking of one of those swinging ball toys, Newton’s Cradle, this law is what you see when you pull back one ball and let it go. The action (one ball swinging into the group) has an equal and opposite reaction (one ball swinging out of the group). Pull back two balls and let them go, and now two balls swing out the other side.

Hope that helps.