What does `if __FILE__ == $0` mean in Ruby

# Only run the following code when this file is the main file being run
# instead of having been required or loaded by another file
if __FILE__==$0
  # Find the parent directory of this file and add it to the front
  # of the list of locations to look in when using require
  $:.unshift File.expand_path("../../", __FILE__)  
end

Note that this particular practice (adding to the LOAD_PATH directory) is not usually necessary with the advent of require_relative, and expand_path is a simpler way of finding the parent directory.