The modulo operator % gives you the remainder after division. When you write 10 % 3, Java divides 10 by 3 (which is 3 with a remainder of 1) and returns that remainder.
System.out.println(10 % 3); // 1
System.out.println(8 % 4); // 0
System.out.println(5 % 2); // 1
You'll use modulo often. Checking if a number is even: n % 2 == 0. Wrapping an index around an array: index % length. Extracting the last digit of a number: n % 10. If the result is 0, it means the first number is evenly divisible by the second.