As close to the earth that they are, gravity is a huge factor. You couldn’t, for example, step outside and just float away into space.

The reason it seems to be weightlessness is that the ISS and the astronauts inside are all falling at the same speed. The forward movement causes and angular movement away from the earth and the gravity pulls downward. This balances out in a wonderful phenomena we called an orbit.

An orbit really is like shooting a cannonball so fast that as it falls to the ground, the ground curves away underneath it to the point it never hits the earth but just goes around perpetually.

For this reason, the space shuttle is falling. It’s also going forward very fast and as it falls goes around the curvature of the earth and just goes round and round, along with the people and things inside it. They all are falling at the same rate, giving the impression of weightlessness.

That’s why it’s called microgravity and not weightlessness.