A common denominator is when the denominators (bottom number) of two fractions are the same. Fractions must have a common denominator to be added or subtracted.
A common multiple is when two numbers can be multiplying to the same number. Ex) A common multiple of 3 & 4 is 12
A common multiple of 6 & 2 is 18
You would need to know the common multiples of numbers in order to make fractions have a common denominator.