The han is wooden board, that is hung outside the meditation hall. Many of the traditional han are inscribed with the Chinese characters inserted here into the picture.
The meaning of those characters is - roughly translated:
Birth and death are important things.
Make use of every moment.
Everything changes quickly.
Time does not wait for humans.
The han is struck three times a day. The first time at daybreak, secondly in the evening, and finally during the closing ceremony. A wooden mallet is used to strike the familiar 7-5-3 pattern with the accelerating rolls in between. The sound is harsh and somewhat unsetteling, trying to remind us of the very reasons we came to study Zen.
In Japan monks hit the han with all their strength, just like they try with their whole being to get through all the delusions and attachments. Traditionally when the han finally shows a hole in the middle the monks have the day off, until a new board is hung.