What does `<<<_HTML_` do in php?

Question

Reading a PHP book and there are a lot of examples with print <<<_HTML_ but cant find any info on it. What does it do? Is it better to print without <<<_HTML_? is it a good practice to use it like in this example?

print <<<_HTML_
<form method="post" action="$_SERVER[PHP_SELF]">
Your Name: <input type="text" name="user" />
<br/>
<button type="submit">Say Hello</button>
</form>
_HTML_;