On White Day, the men repay the girls for their gifts by giving them something white. The gift can be ribbons, candy, jewelry, cookies, etc. Usually it's of more value than the original gift. The general rule is that it has to be two to three times the value of the Valentine's Day gift.
Since Valentine's Day was such a lucrative event, sales people decided to create a second holiday. Since Valentine's Day was for women to buy things, White Day was created so the men would have to buy gifts as well.
Although the origins of the holiday aren't very romantic, it's still a good day for couples... unless you're the guy who received obligation chocolate. Even if you were an afterthought, you still have to give something in return as thanks.