The force of lift, which enables an aircraft to fly, depends directly on the density of the air. Performance levels are always established according to a reference called the standard atmosphere. Flight conditions being rarely standard, one has recourse to the density altitude in order to estimate the standard altitude (ISA) that has the same value as the density of the air you are in.
Click and rotate the pointers on the dials. You can also click and drag the control on the altimeter setting.