What does the defer attribute of the SCRIPT tag do?

A quote from the W3C HTML 4.01 recommendation:

"defer [CI] When set, this boolean attribute provides a hint to the user agent that the script is not going to generate any document content (e.g., no "document.write" in JavaScript) and thus, the user agent can continue parsing and rendering."

To use it, add it as an attribute of the tag:

<script 
  language="JavaScript" 
  src="somescript.js" 
  defer>
</script>