A furlong is a distance which is one-eighth of a mile i.e.: there are eight furlongs in one mile. A furlong measures just over 201 meters or 660 feet. The term furlong is not commonly used these days except in horse racing.  

Before tractors were invented horses and ploughs were used to plough  fields. Furlong is a mixture of two words, furrow and long. A furrow is the trench made by a plough in a field. The word furlong was used generally to describe the length that a furrow was ploughed before the horses were turned.