The player who misses selection in the final 11 but is still part of the team. Can replace one of the playing 11 as a fielder but cannot bat or bowl.
Traditionally, the first substitute player who fields when a member of the fielding side is injured. In Test matches, twelve players are named to a team prior to the match, with the final reduction to eleven occurring immediately prior to play commencing on the first day. This gives the captain some flexibility in team selection, dependent on the conditions (e.g. a spin bowler may be named to the team, but omitted if the captain feels that the pitch is not suitable for spin bowling).
Not even remotely coincidentally, TwelfthMan is the official membership of the England and Wales Cricket Board. Howzzat!