% (modulo)

Calculates the remainder when one integer is divided by another. It is useful for keeping a variable within a particular range (e.g. the size of an array).

result = dividend % divisor

dividend: the number to be divided

divisor: the number to divide by

the remainder

x = 7 % 5;   // x now contains 2
x = 9 % 5;   // x now contains 4
x = 5 % 5;   // x now contains 0
x = 4 % 5;   // x now contains 4

/* update one value in an array each time through a loop */
 
int values[10];
int i = 0;
 
void setup() {}
 
void loop()
{
  values[i] = analogRead(0);
  i = (i + 1) % 10;   // modulo operator rolls over variable  
}

The modulo operator does not work on floats.

* division

Source: arduino.cc