It just means how far are they from getting maximum stats in every single category.
This is obvious in equipments with one single stat, such as MA for Powerful Arcs, the maximum is 123, if you have one with 123 MA then you reached 100% potential. 122 MA is 99%, and so on.
In equipments with more stats, all stats are taken into consideration. You may have something with maximum stats in 3 categories, and another just close to the maximum, it will be at 99% or so.
You can have equipments with a potential higher than 100%, that means that the stats are overall better than the default maximum. There are Powerful Arcs with 130+ MA, for example. The way to do this is through the use of tempering gems, which increase the maximum allowed upon tempering, though it does not increase the probability of getting a successful temper.
Recette - Witch 400 | EmiriTachibana - Diva 400 | SakuraHiiragi - Champion 35x | AnaJessicaAyano - Soul Master 26x | LeslieSun - Witch 26x
Bye Bright Shadow Japan, I will miss you all and my bunch of characters very much ~~