Most styles have different ranks or grades. These grades are represented in different ways. Most follow some type of color scheme. In the style I practice we have 12 color belt ranks or kyu grades. All start at white, then go to advanced white, yellow, advanced yellow, ETC all the way to black belt. Yellow is a plain yellow belt, advanced yellow is a yellow belt with a black stripe thru it. Sometimes, there would be a black stripe at the tip of the belt, or black bar, as you seem to have described. It sounds like your son's class mate is one grade further along than your son if his yellow belt has a black bar or stripe on it. Every school is different however, so this is just a guess based on what you said & what I have seen in the martial arts.