What is <<<__HTML, <<<HTML

In some of the core plugins, I see some html code wrapped with `<<<__HTML` or `<<<HTML`, and also `<<<BODY`. My question is that what is the difference between them? Do we have to use these signatures when adding plain html? Thanks.

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking