The triangle and two defense in basketball is more commonly known as a "junk" defense because it is unconventional as it combines both man-to-man and zone into one half court defense. Two defensive players guard two offensive players shadowing them man-to-man while the other three defensive players usually align in a triangle shaped zone with one playing high up near the top of the key and the others below closer to the paint. The man-to-man defenders guard the two best players on the offensive side neutralizing them best as possible while the others help with the two players as well as keep the others at bay.
The triangle and two is a fairly unorthodox defense so it leaves the offense and opponent at a disadvantage because it's difficult to prepare to play against it. Another advantage of using this junk defense is controlling the tempo of the game by causing the other team's best scorers to tire and work harder to score because the defense is very much aware of them wherever they are on the court. Finally, the triangle and two forces the offense to adjust and move away from its original gameplan in some cases leading to an advantage for the defensive team using this easy to learn defense to frustrate the offense.
The triangle and two, like every defense, has its holes. The middle of the court, as well as the perimeter, is fairly open so teams that can effectively move the ball into either of these areas, or both will find easy success. This happens especially since there are only three defenders focusing on everyone on the floor at any given time. Shooters can breakdown any defense if they make shots however the triangle and two may not be effective at all if one of the best offensive players is a post player. This would clog the paint and make the defense even more susceptible on the perimeter.