The six titular Vs refer to Viridian, Violet, Vitellary, Vermilion, Verdigris and Victoria, the crew of a spaceship involved in a teleportation mishap that results in them being separated throughout space. You play as Viridian and have to explore your new surroundings and find your way back to your ship to reunite with your crew.
Essentially, VVVVVV is a tough 2D platformer with an unusual "flip" mechanic - hit spacebar to switch between walking on the floor and the ceiling (it's in space, after all!) - and a surprisingly engrossing story.
There's a two-level demo to sample at Kongregate, while the full version can be purhcased from Terry's site. I thoroughly recommend it.