Essentially, they are thieves who, while still having the main job of stealing items/money, now have the ability to not only properly defend themselves, but even fare well in some offensive situations.
Thief Fighters are well-rounded fighters compared to their weaker predecessors. If they stick to cover, they can very well take down even the toughest of opponents.
Early translations of FE5 translated Thief Fighters as Rogues after the later class. This is arguably fitting as the two classes look similar and fulfill the same role.