A Tattercoat, or Ragcoat, is basically just a shirt or coat (or maybe a waistcoat [US: vest]) with rags or tatters sewn onto it. The tatters may be wide or narrow, long or short, sparse or dense, square, rectangular or fancy-shaped and in absolutely any colour scheme or none.
It was originally the costume worn by English agricultural workers for the traditional mumming plays, and sometimes for the morris dances, that they performed to make extra money at slack times in the farming year when their usual wages might be reduced or not paid at all.
They were very simple and cheap to make, using materials readily available, but even at the most basic performed the function of turning ordinary clothes into 'Costume'; more elaborate versions could be quite spectacular, especially when combined with headdresses, masks or even just faces blackened with soot.
There are pictures of modern mummers at the National Drama Research Group's Gallery , many of them wearing tattercoats: the ones sported by the Long Company are particularly fine.
Since being adopted by the influential Shropshire Bedlams morris side in the 1970s, they have become the standard kit for morris sides who dance in the 'Border' style and are used by many other sides as 'keeping warm between dances' wear. They can be very cosy if the tatters are packed densely enough! There are more close-ups here or you can go back to the instructions