An altimeter tells a skydiver how high they are above the landing area. Altitude awareness is the single most important habit in the sport, because almost every decision on a jump is driven by height.
Visual and audible
A visual altimeter — analogue or digital — is normally worn on the wrist or mounted on the chest strap where it can be read at a glance during freefall.
Audible altimeters sit inside the helmet and sound tones at set altitudes. They are a supplement to a visual altimeter and to your own awareness, not a replacement for either.
They read height above the ground
Skydiving altimeters are set to read zero at the landing area rather than at sea level. Landing somewhere other than the intended area — a different elevation — is one reason off-field landings deserve care.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.