UT stands for "utility player" in softball and various other sports. A utility player is a member of the team who can play multiple positions on the field as needed.
A UT in softball is an extremely valuable asset to the team, as UT players are able to fill in for other players for various reasons. Utility players typically have an advantage when anticipating plays on the field, because they have played in those positions. Outfielders are the generally the most common type of utility players, as they shift in the outfield and can move into roles in the infield.