The pain threshold is the point at which sensation becomes pain. For example, imagine someone is tapping on your arm and taps progressively harder. Eventually, the tapping will become hard enough to hurt, and that is when it has reached your pain threshold.
Pain thresholds vary from person to person and are abnormally low in fibromyalgia (FMS). That's why things that aren't painful to most people can cause pain in those with FMS. Some research also suggests low pain thresholds are a part of chronic fatigue syndrome, and at least one study shows that pain thresholds drop following exercise for people with this condition.
Pain threshold is distinctly different from pain tolerance.