The common multiple of two or more numbers which has the lowest value. Example: 3 has the multiples 3, 6, 9, 12, 15, 18, 21, 24, etc. 4 has the multiples 4, 8, 12, 16, 20, 24... and 6 has the multiples 6, 12, 18, 24, 30...
The common multiples of 3, 4 and 6 include 12, 24 and 36. So the lowest common multiple of 3, 4 and 6 is 12.