U-15 stands for "Under 15". A U-15 idol is a gravure idol, but is at or under the age of 15. They can be seen most often in publications like Pure Pure and LemonTeen. At such a young age, there is of course controversy among these young idols. They can be photographed wearing swimsuits (including bikinis), but lingerie or underwear is a strict no; photographers and viewers both of this sort of material can be jailed.
These idols are also nicknamed lolita, but this has little to do with the fashion attire.