This is an image of Newton’s Cannon. Given a high enough elevation, shooting a canon-ball at a high enough velocity will cause it to travel further and further until it just goes into orbit, falling around the earth.
That is what’s happening inside the ISS. It’s called free-fall – the same as Newton’s canon-ball.
The ISS is falling toward the earth. The only reason it doesn’t crash to the ground as it falls is because it’s also moving horizontally so fast that as it falls, the earth curves away and they just keep missing, again and again, around and around. That’s called an orbit.
This is why we don’t say the astronauts on the ISS are weightless. We say they are in microgravity.
As an example, let’s say you get into an elevator at the top of the tallest building in the world, and the elevator drops in free fall. Fun! You would fall at the same speed as the elevator and to an observer you’d appear to be weightless. In fact, if you held an orange, it would appear to float out of your hand.
That would end tragically, however, when the elevator hit the ground. Not fun anymore!
That tragic end doesn’t happen to the ISS because the ground just curves away under them and they keep missing Earth! Inside, however, the oranges float around and the astronauts float because the ISS and everything inside it is falling at exactly the same rate.
Therefore there is gravity up there – almost the same gravity that we experience on the ground. If the spaceship could stop moving forward and just hover somehow (it can’t) then everybody would be able to walk around just like we do here on Earth. It’s only because the ISS is in free-fall around the earth that the people and things inside appear to float.