This is a diagram or graph where the 2 axis are height and velocity.
In the simplest explanation, the H/V curve is a diagram where the shaded areas should should be avoided when possible as the average pilot may be unable to complete an autorotation without damage. The H/V curve will usually contain a take-off profile, where the diagram can be traversed from 0 height and 0 speed, to cruise without entering the shaded areas.