The least common multiple of two numbers is the smallest number that both divide into with no remainder.
For 4 and 6: multiples of 4 are 4, 8, 12, 16, 20, 24... Multiples of 6 are 6, 12, 18, 24, 30... Common multiples are 12, 24, 36... The smallest is 12.
We write lcm(4, 6) = 12. This number tells you when two repeating cycles sync up.