It is a very good working practice to comment your code in HTML Comments serve two major functions:

As notes and reminders

Comments allow you leave yourself notes as to what sections of code do, and what they are there for. It is common as a developer to return to code that you have not seen in six months or more: comments allow you to quickly remember what each piece of code does.

As “traffic cones” around code that is problematic

Code surrounded by comment markup immediately ceases to work. Because HTML code is read from the top down, you can narrow down the cause of problems by removing sections from execution by means of comments, moving the start and ending comment markup around to analyse problems via a process of elimination.

Content, markup and code within comments does not appear on a web page, but can still be read by anyone what access to your source code. (In HTML that is as easy as using View / Source in the browser).