The 1.3.8.1.sp1 update on the SimpleScripts side is a combination of all the patches released by ZenCart for the 1.3.8 branch up to this point.

Please see http://www.zen-cart.com/forum/showthread.php?t=131115 for more information on the patches.

If anything breaks during the upgrade (which should take perhaps a second or two to apply) there is a one-click restore feature for any files SimpleScripts changed during the upgrade, and you'll want to go over the patches in more detail if that happens. Most of them are one line fixes, but very necessary for the security of your site.

Hope that helps!