Woollen items of clothing, like socks and jumpers, often get holes.  The heels of socks and the elbows of jumpers are often the first areas to develop holes.  In the past, people took good care of their clothes as they did not have many.  When a sock, for example, developed a hole, it would be "darned" rather than thrown away.  In darning, wool or thread is woven across a hole until it is filled in.