Although Valentine's Day is of European origin, it began to be celebrated in Japan and some other Asian countries in the 20th century, mainly because chocolate manufacturers and other businesses marketed it to the public. This may be why, in Japan, the day is largely about chocolate even today.
Unusually, in Japan it is now the custom for girls and women to give chocolate or sweets to men on 14 February. This may be a small gift to a colleague, friend etc or a special box of chocolates to declare love for a particular man or boy. Then on 14 March, on a festival known as White Day, men are supposed to buy similar gifts for the women who treated them on Valentine's day.
It can get quite complicated, with different types and quality of chocolate being given depending on how special the person is to you. Some women have even had chocolate models made of their faces to give to the men in their lives.
You can learn more about this custom here.