What is a common multiple?

Answer 1

A common multiple is a number that two or more other numbers can both go into. For example, a common multiple of 2, 4, and 6 would be 12.
Not sure what you mean by a "comman" multiple, but a commonmultiple of a set of numbers is a number that is a multiple of each number in the set. That is, it is in the multiplication table of each of the numbers in the set. Note, though, that for this purpose, the multiplication tables do not stop at 10 times or 12 times.
All numbers have multiples. Some numbers have some of the same multiples as other numbers. These are known as common multiples.

18 is a multiple of 6.

18 is a multiple of 9.

18 is a common multiple of 6 and 9.
All numbers have multiples. Some numbers have the same multiples as other numbers. These are called "common multiples."

12 is a multiple of 3.

12 is a multiple of 4.

12 is a common multiple of 3 and 4.

Answer 2

That refers to a number that is a multiple of every number in a set. Usually the numbers are assumed to be natural numbers. For example, 48 is a common multiple of 8 and 6, since it is a multiple of both. In general, the product of two numbers is a common multiple of both - though not necessarily the LEAST common multiple (in this example, the least common multiple is 24). All common multiples of a set of numbers are multiples of their least common multiple. For example, 24 is a common multiple of 8 and 6, therefore 48 (= 2 x 24) and 72 (= 3 x 24) are also common multiples of both.