Recall that a number is divisible by another if you get a remainder of 0. For example, 15 is divisible by 3 because the remainder is 0 when you do 15/5. In other words, 3 goes into 15 five times, or evenly.
In general, if you can put number x into number y evenly, number y is divisible by number x.
For example, 2 goes into 8 evenly, so 8 is divisible by 2.