In astrodynamics a Δv or delta-v (literally “change in velocity”) is a measure of the amount of “effort” that is needed to change from one trajectory to another by making an orbital maneuver. It is a scalar that has the units of speed.

The rocket equation shows that the required amount of propellant dramatically increases, with increasing delta-v.

Therefore in modern spacecraft propulsion systems considerable study is put into reducing the total delta-v needed for a given spaceflight, as well as designing spacecraft that are capable of producing a large delta-v.

Redditor Ucarion via Geekologie for the full article.
Quoted text an explanation of Delta-V via Wikipedia.
Bottom image a more conventional Delta-Vs map from Wikimedia Commons.