What does lcd mean in math?

LCD stands for Lowest (or Least) Common Denominator.

The LCD for a set of two or more numbers is the least number such that each member of the set is a factor of the LCD.

So, for example, given the set {2, 4, 5},

LCD{2, 4, 5} = 60 since 2*30 = 60, 4*15=60 and 5*12 = 60. No non-zero number smaller than 60 is a multiple of all three members of the set.