Hi there. That's an indicator of the type of html that's being written. If it's only displaying "p" it means you're writing regular text. If you're writing bold it will display "p >> strong", etc. You'll notice it only appears in the visual editor as in the text editor you can see the actual html.
It's a handy feature to check that you're correctly formatting the text without constantly having to switch to the text editor, but otherwise it's not something you really need to worry about.
See here for a bit more info on using html in your posts: http://en.support.wordpress.com/beginning-html/