5-Hydroxytryptophan, or 5-HTP for short, is an amino acid and is involved (as a precursor) with serotonin. Serotonin has a direct effect on mood, anger, appetite, and more.
The mountain of research supporting 5-HTP as a mood enhancer is indisputable. It is recognized across the board as being effective in this regards. Diet pill companies include 5-HTP to help elevate your mood. Some believe that feeling better about oneself helps in weight loss and can have a positive effect on your workouts.
Will the better mood that is induced by the 5-HTP cause you to lose weight? No. But can it have a positive effect when combined with other weight loss supplements to accompany your exercise? Sure.