Kabuto is Samurai's helmet. We usually display it on May 5
because it's a day for Children's day (Kodomo-no hi in Japanese),
which is one of Japanese national holidays.
So this is a one of traditional custom in Japan,
and it lasts since the Edo period (400years ago).
Kabuto is the symbol of important protector to safe one's life
and also there are wishes to protect boy's life from accident, illness and disaster. It's used to pray for little kids' growing.