Comments allow code to be ignored. In the HTML language, comments are done by wrapping the code you wish to ignore with <!-- and -->
<!-- This text would not show up as it is a comment -->
You can easily uncomment by just removing the comment elements.
Note: Be careful when uncommenting HTML to remove both the beginning and end portions of the comment!
Note: Comments might be done differently in other languages such as JavaScript, but the general concept of removing whatever marks a comment applies.