RTD is an abbreviated term for retired. It indicates that the referee stopped the bout during the minute rest period between rounds. He may have done so on his own, per the ringside doctor's judgment, or when a fighter indicated during the rest period that he did not want to continue on to the next round.
So, for example, if a boxer quits during the rest period after the third round, BoxRec will record it as RTD-3.
RTDs are also recorded as technical knockouts (TKOs).