Blog post? Do you mean fence post?
As Thomas Jefferson might say, I am an old gardener but a young blogger.
--What's the URL, or Web address? You should only care about this if you have a blog of your own you'd like people to know about. If so, fill it in and your name will be linked with your address -- a bonus for your own blog. Otherwise, just leave it blank.
--Do I have to include my e-mail address? Nope. Just leave it blank if you like. Fill it in if you'd like me to be able to contact you, but rest assured I won't share the address without your permission. It might be handy to include in case a gardener wants to contact you privately about an issue you've raised, and you want me to be able to pass on that person's e-mail address without identifying yours. You will also need to leave your email address if you'd like to win one of my prizes! I'll need to know how to contact you.
--Do I have to put in my full name? Not if you don't want to. You can use your full name, your first name, a fancy nom de jardin or no name at all. Just comment.
--What's this funny letter I have to put in the box before posting? Is it important? Yes; it helps us weed out spam. And don't forget to hit "Post" once you've filled it in.
As for the site owner approving the comment before it's posted, that's me. I read over a comment to make sure it a) isn't computer-generated spam, b) doesn't contain obscenities, c) doesn't involve personal attacks on other posters. (I'm fair game, but I do prefer you make your criticisms in a constructive way.) I don't edit it for spelling or grammar. Then I post it as quickly as I can.