editing?

Hi, I just opened my blog, not in my dashbord but in the published version. I checked my first post and nóticed that at the end of this post there was a button that said “edit”. I clicked this button and to my surprise I could edit everything!
I was in my dashboard. So if I can come in my dashboard anybody can.
Please tell me what I did wrong and what I have to do to fix it.

Thanks in advance,
Roy