A player who holds positions at at least two different places on a team. This is generally a person who possesses extra talent or can fill in quickly at various positions due to their athleticism.
Tweeners are special athletes that may not necessarily fit solidly into any single position, but the team wants to keep them around because they can easily be inserted into multiple positions.
It can also be the case that the player is so talented at many positions that they are routinely rotated around. Special teams players are generally tweeners because they specialize at a special teams position, but also play back up to defensive or offensive players.