LCM or Least Common Multiple is the smallest possible integer that can be divided by two numbers considered.. It is very much useful especially when dealing with fractions since it is used in finding the LCD or the Least Common Denominator..
Say you are asked of the LCM of 3 and 4
3 = 3,6,9,12,15,18,...
4 = 4,8,12,16,20,...
notice that the first multiple and the lowest for that matter is 12 which also happened to be LCD if it's in a fraction form.. let's take another example
Say you're asked of the LCM of 9 and 10
9 = 9,18,27,36,45,54,63,72,81,90,99,108
10 = 10,20,30,40,50,60,70,80,90,100,110
notice that the first multiple which is common to both 9 and 10 is 90 therefore 90 is the LCM of 9 and 10
