LCM means Least Common Multiple.
For example, we are given the numbers 4 and 5 and we need to list the multiples of each one, like so:
Multiples of 4: 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44....
Multiples of 5: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50....
They both have 20 and 40 in common in terms of multiples. The LCM asks for the LEAST common multiple, and 20 is less than 40, so 20 is the LCM in this case.
