Sometimes you want to leave a note in your HTML that the browser completely ignores. Maybe to explain why something is structured a certain way, or to temporarily hide a piece of code without deleting it. That's what comments are for.
<div>This shows up on the page</div>
<!-- This does not show up -->
<div>This also shows up</div>Anything between <!-- and --> is invisible to the user. The browser skips it entirely.
A common use case: you're working on a component and want to test it without a certain section. Instead of deleting the HTML and losing it, you comment it out:
<div>Account Details</div>
<!-- <div>Related Opportunities</div> -->
<div>Contact Information</div>The "Related Opportunities" div is still in your code, but the browser won't render it. Remove the <!-- and --> when you want it back.