Let’s take an everyday example.

We are riding in a car, speeding down the highway at 70mph. If we toss something (a ball?) softly up in the air, does it land right back in our hand? Or does it go spat against the back windshield, because when it lost contact with our hand it is supposed to fly backward at 70mph relative to the car?

So how does it work?

When the car is driving. Everything in the car, you, me, the ball, the air inside the car, are all going forward at the same speed. When we toss the ball upwards, we give it an upward force , but we didn’t change its forward speed. So the ball still kept going in the forward direction at the same speed as before, which is at the same speed as your hand. That’s why it can fall right back into your hand, as if the car was still.