Checkbox object in HTML has a property checked as boolean - it has true or false, so javascript can check its status easily.

<script>
function myFunction() {
  // Get the checkbox
  var checkBox = document.getElementById("myCheck");
  // Get the output text
  var text = document.getElementById("text");

  // If the checkbox is checked, display the output text
  if (checkBox.checked == true){
    text.style.display = "block";
  } else {
    text.style.display = "none";
  }
}
</script>

Checkbox: <input type="checkbox" id="myCheck" onclick="myFunction()">

<p id="text" style="display:none">Checkbox is CHECKED!</p>