The effective gravity inside the ISS is very close to zero, because the station is in free fall. The effective gravity is a combination of gravity and acceleration.
If you're standing on the surface of the Earth, you feel gravity (1g, 9.8 m/s2) because you're not in free fall. Your feet press down against the ground, and the ground presses up against your feet.
Inside the ISS, there's a downward gravitational pull of about 0.89g, but the station itself is simultaneously accelerating downward at 0.89g -- because of the gravitational pull. Everyone and everything inside the station experiences the same gravity and acceleration, and the sum is close to zero.
Imagine taking the ISS and putting it a mile above the Earth's surface. It would experience about the same 1.0g gravity you have standing on the surface, but in addition the station would accelerate downward at 1.0g. Again, you'll have free fall inside the station, since everything inside it experiences the same gravity and acceleration (at least until it hits the ground).
The big difference, of course, is that the ISS never hits the ground. Its horizontal speed means that by the time it's fallen, say, 1 meter, the ground is 1 meter farther down, because the Earth's surface is curved. In effect, the station is perpetually falling, but never getting any closer to the ground. That's what an orbit is. (As Douglas Adams said, the secret of flying is to throw yourself at the ground and miss.)
But it's not quite that simple. There's still a little bit of atmosphere even at the height at which the ISS orbits, and that causes some drag. Every now and then they have to re-boost the station, using rockets. During a re-boost, the station isn't in free fall; instead. The result is, in effect, a very small "gravitational" pull inside the station -- which you can see in this fascinating video.