You don't normally calculate the lcd of a single number (fraction). You would normally take the lcd of two or more fractions. In that case, ONLY the denominators are relevant to find the lcd.
the numerator is 2 and the denominator is 5
Denominator = 5, numerator = 18
LCD of 5, 9 and 2 = 90
The numerator is 5 (five) and the denominator is 12 (twelve).
The numerator is how much of the denominator. For example, if you have 3 green apples and 2 red apples, the total amount of apples is the denominator, 5. so, 3/5 of the apples are green, and 2/5 of the apples are red.