The least common denominator is used when adding or subtracting fractions. Fractions consist of a number on top and a number on the bottom. The bottom number is the denominator. To add or subtract two fractions the denominator must be the same. For example, if you were adding 1/3 plus 1/2 then you would need to change the denominators of both fractions to 6 and then you would have 2/6 plus 3/6 which equals 5/6.