It's exactly what it sounds like, too: If someone types in an HTTP address to get to your blog, this sends them to a secure version instead. Otherwise, they'll have the choice of browsing securely or not should you not opt in. This could cause a few hiccups if you have a fancy template or add-on running on your blog, but the folks in Mountain View have a warning tool for anything that might run into issues following the changeover. And just like that, your blog's visitors can browse a little more safely.