GOOOOOOD FREAKIN QUESTION!
Its hard to say what you should know. Laws are wordy and complex and horrible, why we decided on optometry and not law school haha.
I think a general knowlage about the subject should sufice for a pre-opt interview. Just know that laws regaurding scope of practice vary from state to state, IE what proceedures you are allowed to perform. In some states legislation dictates what kind of insurance you can bill for, ex wording might exclude opts from billing medicade; therefore prohibiting opts from seeing medicare patients unless they paid out of pocket. It would be a good idea to be knowlageable about non-discrimination practice bills/scope of practice laws/and where the profession is going in the state your from/school you want to go to/and state you want to practice in.
Now finding this info online is kinda hard, so best bet is to talk to your OD about the particular laws of your state. Also be aware that this whole healthcare deboggle is a huge issue, depending on its wording it could affect what patients opts can see, so there is a huge lobby effort by the AOA to make sure the healthcare bill is worded correctly.
All this is probably overkill for the sole purpose of your interview. BUT it nevertheless is VERY important regaurding your decision to persue optometry. So at least know a little about it, you will impress the hell out of your interviewer if you do.