Imagine you can't see or hear anything but you can read a compass, and you have an internal map of your house and neighborhood. You also know how long your steps are with some amount of accuracy. You would probably be able to get out of your house and maybe to the corner store, but the inaccuracies in your compass and distance estimation would add up over time, and on a long walk you might overshoot the sidewalk and walk down the middle of a busy street by mistake.

They know where you took off from, and can detect your movement with precision.

It's knows where it was and where it isn't

They use gyroscopes and accelerometers to measure the aircrafts movement from the starting position at takeoff. That can then be used to plot the course the aircraft has taken to show the current location.

https://en.m.wikipedia.org/wiki/Dead_reckoning

First, they have to align on the ground. You initialize them with your current known position (usually by GPS or your known airport/gate spot). Then, you wait for them to synchronize with the Earth's rotation. If you're far north, like in Alaska, this could take half an hour. If you're close to the equator, it could take 5 minutes. Once they're ready, from that point, any movement you make, it will know where you are and where you've been.

If you spin up a gyro and begin moving around, it will maintain it's starting position. You can use this deflection to calculate direction. If you know how fast you are going and for how long, you'll have your position.

Mechanical gyros drift. It's the nature of a world with friction. Newer IRUs use laser gyros, so the only real drift they have comes from extremely minute rounding errors.