When you're busy building your own website or blog it's easy to get caught up in all the products, services, images or bios that you need on the site. Because of this and because it's obvious to you what the site is about you may forget to have a clear statement on your main landing page (or home page). I've been to many websites that have found me hunting around trying to understand what exactly the website is about. Sometimes even the "about" page will be about the website owners and not about the purpose of the website.
Even if you aren't selling a product or a service you need to let the visitor know from the first glance what you are about. For example, if you do a blog on vintage toys or fashion or even just a personal blog. State what it is on the front page or on a sidebar as well or someplace that will show up on each page.