The joule is a scientific unit named after James Prescott Joule and represented by the letter J, which measures an amount of energy, heat, or work. One joule is equal to the amount of work done (energy transferred) when one Newton (another scientific unit, represented by N) of force is applied through a distance of one meter. A Newton is the amount of force required to move one kilogram at a speed of one meter divided by the amount of time in seconds (represented by s), squared.
In other words, J = N * m = kg * m ^ 2 / s ^ 2.
When looking at buying a surge protector, get a surge protector with a joule rating of at least 200. The higher the rating, the better the protection.