This is one of the fundamental concepts in
number theory.
If a, b are integers then a is said to divide b, written a|b, iff there is an integer c such that b = ac. If a divides b then a is said to be a divisor of b, and b is said to be divisible by a.
Some basic results:
a|0 for all a
0|a iff a = 0
1|a for all a
a|b => a|bc for all c
a|b and a|c => a|(b+c)
Every number a is divisible by ±1 and ±a; these are called improper divisors of a. Any other divisor of a is called proper divisor of a. If a has no proper divisors then a is said to be prime.