What does the % operator do in Ruby in N % 2?

In answer to the question "What does the % symbol do or mean in Ruby?" It is:

1) The modulo binary operator (as has been mentioned)

17 % 10 #=> 7 

2) The alternative string delimiter token

%Q{hello world} #=> "hello world"
%Q(hello world) #=> "hello world"
%Q[hello world] #=> "hello world"
%Q!hello world! #=> "hello world"
# i.e. choose your own bracket pair
%q(hello world) #=> 'hello world'
%x(pwd)         #=> `pwd`
%r(.*)          #=> /.*/

3) The string format operator (shorthand for Kernel::sprintf)

"05d" % 123 #=> "00123"

That's the modulo operator. It gives the remainder when counter is divided by 2.

For example:
3 % 2 == 1  
2 % 2 == 0

% is the modulo operator. The result of counter % 2 is the remainder of counter / 2.

n % 2 is often a good way of determining if a number n is even or odd. If n % 2 == 0, the number is even (because no remainder means that the number is evenly divisible by 2); if n % 2 == 1, the number is odd.