Well, I'm a bit rusty but the theory is simple.
Simplified explanation!
The air pressure tells your plane what altitude it holds above the sea.
The altimeter is set according air pressure. Usually above 5000 feet it's not really important, anyway.
When you take off is a good thing to make sure that your altimeter is set so that it reads 000000 feet.
When you set your altimeter you use either a knob or in another fighter plane that defends Europe ;), you directly set the pressure in
Millibar with a keypad.
On the left-Hand Glareshield.
But I digress.
Let's say you want to approach and land on an airfield that has its runway 300 feet above sea level against the one you departed from that was (just an example throwing numbers here, ok?) 2000 feet above the sea.
If your altimeter is still set for the Airfield you departed from when you touchdown the altimeter will read minus several hundreds feet, 1700 more or less. That is if the pressure is the same as the airfield you took off from.
Well, that's when the simplified explanation gets a bit more complicated so I stop here...
In general, obviously, you want to keep the altimeter set and checked out now and then.