NMN is a mononucleotide of nicotinamide, an organic molecule found in all living things. On a molecular level, it is a ribonucleotide that is the primary structure that is part of nucleic acid, RNA. In terms of structure, the molecule is made up of a group, a group of ribose, and a phosphate group. NMN is the primary precursor to the vital molecule known as nicotinamide dinucleotide (NAD+) and is thought to be the most important ingredient in increasing NAD+ levels within cells.
What is Nicotinamide Adenine dinucleotide (NAD+)?
NAD+ is a vital coenzyme essential for cell functions and for life. They are catalysts that enable biochemical reactions. Coenzymes are the ‘helper' substances that enzymes require to function.
What does NAD+ do?
NAD+ is one of the most plentiful molecules within the human body, aside from water, which means that the organism could die without it. Various proteins utilize NAD+ throughout the body, including sirtuins, which help fix damaged DNA. It is also essential for mitochondria which are the main powerhouses of cells and produce the chemical energy our bodies require. 
NAD+ functions as a Coenzyme Mitochondria
NAD+ has a vital role in metabolic processes such as glycolysis and the TCA cycle (also known as the TCA Cycle (AKA Krebs Cycle or Citric Acid cycle), and the electron transport chain, which is present in our mitochondria. It is the way we generate energy in our cells.
NAD+ is a ligand that binds to enzymes and transfers electrons among molecules as a ligand. Electrons are the fundamental atomic unit for energy in cells. By transferring electrons from one molecule to another, NAD+ acts through a cellular mechanism that is similar to charging a battery. The battery's capacity is diminished by using electrons to generate energy. The electrons cannot return to their initial state without an increase. In cells, NAD+ serves as the boost. This way, NAD+ may decrease or enhance levels of gene activity and enzyme activity, and cell-mediated signaling.
NAD+ Aids in Controlling DNA Damage
As organisms age and develop, DNA damage results from environmental factors like pollution, radiation, and insufficient DNA replication. In the present theory of aging that the growth of damage to DNA is the primary reason for aging. Most cells have the “molecular machinery” to repair the damage. This machine consumes NAD+ as well as energy molecules. This is why DNA damage could deplete valuable cell resources. 
A crucial protein for the repair of DNA, called PARP (Poly (ADP-ribose) polymerase), relies on NAD+ its perform its job. Older people experience lower concentrations of NAD+. The development of DNA damage due to aging causes an increase in PARP, leading to a decline in NAD+ levels. This decrease is made worse by the possibility of further mitochondrial DNA damage.