It means they want to see stuff you post, and/or post stuff that (if you circle them back) you will see.
>Do I now see whatever spammy crap they post?
What? No. That would be super dumb.
>Do they see what I post?
If it's public or if you post it to a circle that includes them.
>Why wasn't I asked for my permission?
Because they haven't been granted any special powers by virtue of adding you to a circle.
>I've never met this person and I don't want to see their spam and I certainly don't want them seeing what I post.
Then don't post publicly- choose a more limited audience to post to.
>Who are they and what do they want?
Go to their profile page and find out, if you care. But you probably don't- it's safe to ignore them.