PLEX is 30 days of game time in the form of an in-game product. You can buy PLEX here or from other gamers using in-game ISK. Lots of gamers pay their subscription simply by playing the game and purchasing PLEX on the EVE market.
PLEX is actively traded in between gamers and can be discovered in every significant trade hub of EVE Online. It is just consumed when you trigger it, affording you the liberty to use it when and where you need it most.
Offering PLEX to improve your in-game wallet and purchasing PLEX to pay your subscription using in-game ISK are two of its numerous uses. PLEX can also be used to buy several account services and even to pay for out of game events and products.