Expeditions are actions that can be performed on island hexes, which are the land spaces on the Game Board world map that are neither Altar spaces nor Port spaces.
When a player's ship is on a Game Board hex tile with an island that is not a Port or Altar, and which has not yet been Explored, the player can perform one of two special actions: resolve an Expedition Card or start an Exhaustive Expedition. In either case, after such an action has been finished, place a Explored token on the island hex, and Expedition actions can no longer be made for the island.
Note: the Exhaustive Expedition is one of the "optional advanced mechanics" and is not available when playing the basic version of the game.