The kanji of "setstu" means season and "bun" is division therefore "setsubun" referring to seasonal division, and February 3rd is a division for spring.
On setsubun, we throw and eat roasted soybeans (called "mame-maki") which has become a small traditional event to exterminate a demon (or an ogre or a devil ...), called "oni" in Japanese. ("oni" is usually red in Japan)
So, why do we throw and eat soybeans?
There are theories to it, but here are the famous ones:
*Soybeans have the power to get rid of the evil spirits
*People succeeded in exterminating the demons by throwing soybeans to the eye of the demons
Throwing soybeans has been continued since the Muroyama era (1333-1573).
We call out "oni ha soto, fuku ha uchi" - "Out with the demons, In with good fortune" and throw the beans outside of the windows and inside the rooms.
And afterwards we eat the soybeans, as many as your age. (So every year the number of soybeans we eat increase by one)
On setsubun, we also eat "eho-maki" which is basically a long and thick sushi roll.
"Eho" refers to a lucky direction in Japanese, and every year there is the "lucky direction" and we eat the eho-maki looking at that direction.
The lucky direction of 2016 was south-southeast.
If you're in Japan next year, enjoy mame-maki!!