LCD is the LCM of the denominator of a fraction.
For example
9/2,3/4, 5/6
You have to find the LCD above, to obtain the LCD, list down the multiples of each denominator:
2:2,4,6,8,10,12
4:4,8,12
6:6,12
The moment you saw that each of the numbers has a same number in their multiple, means that it is the LCM, which is 12. So it is considered as a LCD, because you find the LCM(same as LCD) of the denominator.