Note: Larger, more legible version right HERE.
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.
Delta-v is produced by the use of propellant by reaction engines to produce a thrust that accelerates the vehicle.
I hope that made sense, because I'm not explaining it myself. I'm not very good at stuff like that. Or reading subway maps. I don't care if all I need to do is ride the red line from one end to the other, I always get lost and wind up wandering the tunnels on foot. That's how I met the ninja turtles. "The ninja turtles live in the sewers, not subway tunnels." You shut up, this is my story. We had a pizza party. Plus Michelangelo showed me a topless shot of April.
Thanks to Arrogant Tiger, who is so arrogant he is always bragging about how good he would be at driving a subway.