A thimble is usually used by a seamstress to protect the fingers against pinpricks.

A thimble is often used on the ringfinger, so the seamstress can sew without the worry of getting pinpricks all the time. It is basicly a small metal "cup" that keeps the pins from reaching the finger. The small holes stop the needle from penetrating the thimble.

By the 16th and 17th century, the thimble was being mass-produced. For some reason, different people tried to take credit for the invention during this time period. 

However, written sources mentions that the use of thimbles goes all the way back to the 12th century, so it had been around for a long time by the 16th and 17th century. Originally, the thimbles were not made of metal. Instead, people would make thimbles out of leather or horn, since these materials could also protect the fingers against pinpricks.