In general, it can be said that air pressure and temperature decrease as altitude increases. 

This means that if the air pressure drops, it is very likely that the wearer of a watch with an altimeter is travelling to higher altitudes. The reverse is also true: if the air pressure rises, this indicates that the wearer is descending. 
Thus, altitude measurement is the interpretation of air pressure.

Thanks to the continuous measurement of air pressure in the watch, extremely reliable trends and predictions regarding altitude can be derived. 

On the one hand, it can display the so-called absolute altitude. This is done by simply entering the current reference altitude in the watch, allowing the actual altitude to be read off the display.

On the other hand, the altimeter can also be used to display the relative altitude.
To do this, it is sufficient to correct the altitude value in the watch to "0 m" at the foot of a mountain, for example, in order to see how high the user has climbes. 
CASIO watches can display an altitude from -700 m up to 10,000 meters.