NMN (Nicotinamide Mononucleotide) is believed to raise levels of NAD+, which in turn is a vital component of mitchondrial energy production (and levels tend to fall with age).

I'm actually taking plain Nicotinamide (one form of Vitamin B3) in larger amounts (3g/day) and the benefits I feel from it are similar to those ascribed to NMN (the recommended dose of which is about 1g/day).