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.