A han is a wooden board struck by a mallet with a particular rhythm to announce the beginning of a zazen period in a Zen monastery. It is also sometimes struck before the monks retire for bed.
Often, a variation of this verse is engraved on the han:
Let me respectfully remind you,
Life and death are of supreme importance.
Time passes by swiftly and opportunity is lost.
Each of us should strive to awaken.
Awaken! Take heed, do not squander your life.