It's really very simple.  If your site has legal terms that you want your users to accept before registering, then you can throw this plugin in your mod folder and it will add that little checkbox and link to the externalpages plugin's Terms page so that users can read it before checking the box, if they want.  If they do not agree to your terms or conditions, their registration will be rejected.

Requires externalpages (because that is the standard plugin used to create a /terms page in Elgg).