Some of your grain has just been taken by a guard. This is the tax you pay for living in this town. Back in the day, grain was used like currency, so it makes sense that guards went around collecting grain tax.

You have to pay this tax, because if you don't the guard will cut you down with his special German saber, a zeiber (ぜい). "Don't you hear vat I zei? Pay ze tax or I vill zlei you vis my zeiber!" he shouts.