Otherwise it’ll fall back to Earth. The thing about space is that getting there is easy, but staying there is hard. Imagine a cannon placed on the Earth, then fire it horizontally and see where the shot goes. Because vertical movement doesn't affect horizontal motion, a higher muzzle velocity means the ball will go farther before hitting the ground.
But eventually, if you fire fast enough, the ball won't fall and hit the ground, it will miss the ground. That’s how orbits work. The farther away you are from Earth, the less gravity there is pulling the ball to the ground, so you can go slower in those orbits. But the ISS is relatively close, about 90% of the Earth’s gravity acts on it, so it needs to go really fast.
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