simple meaning of Md5 in php

Md5 is hash generator of a string,it is used for encryption in php,it returns 32 character hex numbe.MD5 "compresses" any stream of bytes into a 128 bit value.
Syntax in Md5
<?php
md5(string,raw);
example
<?php
$test = "Hello";
echo md5($str);
?>
it will be above output:
8b1a9953c4611296a827abf8c47804d7