A number is divisible by 3 when there is no remainder from the division of the number by 3. Another way of saying this is that the number is a multiple of 3.

6 is divisible by 3: 6/3 = 2 remainder 0 (6 = 2*3)

7 is not divisible by 3: 7/3 = 2 remainder 1

6 is divisible by 3: 6/3 = 2 remainder 0 (6 = 2*3)

7 is not divisible by 3: 7/3 = 2 remainder 1