The lume of the watch refers to the luminous phosphorescent solution applied on watch dials. In low-light conditions, the watch dial appear to glow. Typically strontium oxide aluminate is used, although older watches may feature radium or zinc sulfide.
Some brands, notably Luminox (see photo above) and Ball, use small gas-powered tubes containing laser-sealed radioactive tritium reinforced with borosilicate; the tritium gives off a steady glow and requires no "charging period" prior to a change in light conditions. Despite the "radioactive" nature of the tritium, it isn't considered hazardous.