In the past, incandescent bulbs were the most widely used lighting source, and their brightness was determined by their wattage, with 100 watts being the brightest and 40 watts being the dimmest.
With LED lights, most of the electricity is converted into light, instead of being dispersed as heat as it is with incandescent bulbs. Hence, a brighter light can be produced without producing heat.
The "Wattage Equivalent" of LED lights is a comparison to the wattage of incandescent bulbs, and serves as a baseline for what a certain LED light is equivalent to in terms of brightness. For example, a 100-watt incandescent bulb would be equivalent to a 100-watt equivalent LED bulb, which typically uses only 10 watts of electricity.