A thimble is a protective device that fits over a finger and is used to assist in pushing a needle through materials when sewing by hand. They can be made of many different materials including metals, woods, leathers, and plastics.
When sewing by hand, the needle needs to be pushed through the materials being sewn. Certainly, with some thicker leathers, this can require a fair amount of force. That force, applied from a finger onto a tiny needle head could be painful or in some cases dangerous, especially over time with repeated stitching.
A thimble covers the finger and provides a harder surface for pushing the needle. So smart! Thimbles are available in different sizes, and different surface finishes/styles, so one can be found that works best with the types of needles and style of sewing being performed. Most leather workers that hand-sew will have one, two, or many needs around the workshop.