How to get ready for a visit to a lab where I would like to do undergrad research? I am a senior physics major and I recently applied to a national lab that is associated with my university. I applied for a student research position through their website (uploaded my resume and wrote a cover letter etc.) I was contacted by the PI of the research team to come for a 'visit' for an hour with him and his team. He went through the hassle of giving out the visitor pass for me.

More about me, I am a senior but I have no research experience. I don't have a strong GPA and I admit I am not that 'smart' like other students. Should I have to explain about my GPA?

So my question is what should I prepare myself prior to my visit? Do you think it would be some sort of an interview? What sort of questions (academical/personal) should I prepare? Also, should I take an updated resume (I don't think it has really changed since I applied)?
You should treat it as an interview unless you get information telling you otherwise.  Yes you should bring a resume, but you may not need it.  You should read about the PI's research, lab, and team.  Usually there is some information on the national lab's website.  Typically undergraduate researchers are not expected to know how to do research, so you might be questioned about your motivations for learning to do research.  If your grades are not good you should be prepared to say what you are doing to improve them.