NAD+ is a carrier of hydrogen atoms between proteins, with NAD+ being converted to NADH, which carries the hydrogen.

“NADH is basically NAD with a hydrogen atom attached to it. NAD+ has a positive charge like the end of a battery. And then if you stick the hydrogen onto the vitamin B3 part of the NAD, then that's going to be called NADH. And that's important in a cell, 'cause that's the other major function of NAD. One is to turn on the sirtuins and DNA repair and all that good stuff. But it also is known as a hydrogen carrier molecule that takes hydrogens and moves it from one place to another.”