Fyrd is an Anglo-Saxon word denoting the armed levies raised by a king for the defence of the kingdom.

The original meaning of the word was that of a 'journey', which came to describe a group of men who made the journey, and then came to have the specific meaning of a force of men on an armed expedition.

The king granted the land, and attached to the land came an obligation to send a representative to serve in the king's forces at the rate of one man for every five hides of land. Each individual normally served for between sixty and ninety days with each hide of land being charged in kind for goods to the value of four shillings towards the maintenance of the representative sent.

There were penalties for the failure to attend when summoned and for desertion and cowardice in face of the enemy, the offender being placed at the king's mercy for all of his land and for his very life .

From information at regia.org