Memory is the ability of the brain to store and recall information. It's so integral to the ability to learn skills, form and develop relationships, and make decisions that it garners a considerable amount of attention in science and from consumers of health products. Perhaps the most critical memory-related issue today is neurodegeneration. As the average lifespan of people has increased, it has become more common to develop neurodegenerative diseases, the most common being Alzheimer's, which negatively impacts memory, among other mental faculties. There are many hypotheses as to its causes, and it remains controversial, but it's clear that damaged and dysfunctional brain cells due to aging are at play.
Even without a full-blown disease, a gradual decline of memory with age is typical, and a vast array of "nootropics" (substances intended to promote cognitive function) have become available to meet the demand for better memory. Even young people, with no apparent decline in memory, seek to enhance their minds through supplementation. This page deals with three instances: in diseases, normal cognitive decline due to aging, and those with normal memory.
Memory is naturally tied to many of the other psychological and physiological phenomena. Anxiety, stress, depression, fatigue, and sleep quality all have an effect on memory. Mitochondrial and metabolic health are also at play because sluggish energy transport and metabolism will likely reduce the brain's processing speed. For that reason, it's essential to be clear about the study population, because the effects of many supplements may only be relevant to people with some health issues, and may not necessarily translate to others.