Checkbox condition using javascript

i need help on javascript condition.

Here is my working fiddle: http://ift.tt/2jVOhli

   $(document).on("click", "input[name='txt1']", function() {
  $('#modal1').show();
});



$(document).on("click", ".btn1", function() {
  $('#modal1').hide();
});

On my Label 1 checkbox, you can have multiple checked. what i want here is that if checkbox checked is greater than 1, it will echo the value "Multiple" inside my textbox.

Please explain to me how your code works so that i can used in the future. Thanks in advance to anyone who can help me.