What does "if (isset($email)):" mean?

[Anon]

What does "if (isset($email)):" mean in the next script?

<?php
if (isset($email)):
$target="[e-mail]";
$subject="My Hero submission";
mail($target,$subject,"Nominated by: ".$MyFirstname."\nCountry: ".$MyCountry."\nE:mail: ".$MyEmail."\n\nFirstname: ".$MyHeroFN."\nLastname: ".$MyHeroLN."\nWhy: ".$Why."\nQualities: ".$Qualities."\nWord1: ".$Word1."\nWord2: ".$Word2."\nWord3: ".$Word3);
endif;
?>

[Anon]

if (isset($email)):