Permalink is just that exact spot on the page where you see that permalink. So, if you click on a permalink next to you specific post someone left in your journal, the url in your browser will point that post rather than your entire page.
This way, if you have a lot of visitor messages and would like to link to one of them in particular for any reason, you can just use the permalink. Here's an example from your own journal:
Kim's post
To respond to a member, you'd have to post on that member's page because they don't get notifications when you post on your own page.