Libido, or sex drive, refer to the spontaneity, frequency, and magnitude of sexual desire. When a persistent lack of sexual fantasies and desire leads to notable distress and difficulties in relationships, that's when it's deemed Hypoactive Sexual Desire Disorder (HSDD). It affects both men and women and is fairly common.
Low libido is a combination of physiological, psychological, and social factors, and varies in its cause from person to person. Fatigue, depression, and stress/anxiety are commonly cited reasons for low libido. Dysfunctional signaling in the dopaminergic reward system of the brain, or an excess of inhibitory neurotransmitters such as serotonin, prolactin, opioids, and endocannabinoids, particularly from drug abuse, is a reason for low libido. On the social end of things, strained relationships may be a common cause, and in that case, supplements will be unlikely to help.