Mocha testing difference between done() and done as paramter to function

please explain difference between done() method and done keyword passed as a parameter to a function?

    it("qwerty",function(done){
------
------
done();
});
it('qwerty', function(done){
        ----------
        ----------
        .expect(404, done);
      })

Please explain the differences and how many times i can call done() in loop if i am calling like 15 times i am getting an error "multiple times done() called"