Dobok is the special suit worn when practicing taekwon do, one of the most known martial arts today.
The suit itself is white until you receive the grade "first dan" (black belt). Then your suit will be decorated with different black stripes and Korean icons. But before you become first dan of higher, you must grade from white belt, to green, to blue and to red. These grades before first dan (black belt) is called "gup" from tenth to first. Tenth gup is lowest and therefore white belt, and first is highest (red). It is not possible to "leap" over any of these colors, and therefor you should not make fun of those with yellow belts!