In many martial arts, a white belt needs to learn the basics, general terminology and be able to go through basic training. In American Kenpo Karate a white belt needs 3 months to test for yellow belt. These are the basic requirements for a white belt to test for yellow belt. (These requirements might be different from your school, style and association)
There are not many requirements for white belts because they are discovering the world of martial arts and they do it with pretty simple moves which they will apply in the yellow belt.