Pilot License Extensions (PLEX) are an in-game item that represents a single month of subscription time.
It is possible to purchase PLEX both through CCP's Account Management site (with real money) or through the in-game market (with ISK). All PLEX sold on the in-game market have been purchased by players and redeemed into the game as physical items, so it is possible to purchase and sell PLEX as a method of turning real money into in-game currency.