Digital gyroscopes are just like those spinner toys you've seen when you're a kid, where you pull the string and you can balance the whole thing on your fingertip easily. The ones in chips don't have metal rings, instead they use very very small springs that help it measure rotation.
Compared to accelerometers and even magnetometers, digital gyros are surprisingly complex, but thanks to their existence we can match them up with other sensors to create orientation sensors. Magnetometers are too slow and are easily messed up by magnets and accelerometers cannot measure spin along the gravitational axis (they can only measure tilt) so give a hand to the gyroscope!
When gyros are manufactured, they have some zero-offset error, much like magnetometer hard-offset error, that can make measurements difficult. It's easy to detect and remove this offset, we just take many measurements and look for the 'offset' from zero
For example, here's a common MPU-6050. If we put it down flat on a table, and take measurements, we will see that neither X, Y or Z (red, green, blue) are at zero. Even though its not moving! That's the zero offset. In this case, its about -0.43 for X, 0.32 for Y and 0.34 for Z.
Once calibrated, you can see that there's still a little noise (there always is!) but its only 10% as much as the offset, so we'll get much better measurements. We can try to improve the noisy measurements with filtering if necessary.