The Void is an unofficial term for the location of ships that have been removed form the game. The term comes from the alien Void, which was the original and most common cause of ships being removed.
Once a ship is removed from the game, it remains as such permanently and cannot come back into the game. Certain effects (such as Healer) can rescue ships just before they are removed, but once they are removed there is slim if any chance of getting them back.
In the current edition, all game effects that can remove ships from the game state that they cannot cause a player to have fewer ships than the number of foreign colonies needed to win the game, and usually send the ships to the warp instead if the player would be reduced to fewer than that number of ships. In previous editions, this rule was much meaner: game effects that could remove ships from the game instead stated that if a player was reduced to below this amount, he or she had to discard his or her hand and was eliminated from the game entirely.