This means you can't perform some site functions, like reply to posts, or PM other users.

To correct this, go to your profile page and check your email address, then click the verify link.

If your email address is wrong, click Edit Profile (also in the Tools box on your profile page) and correct it.

If your email address is correct and verified (green tick beside it) then you simply need to log out, then log back in to fix the problem.