Personally I feel this says either one of two things about the company.
1 They are either too lazy to schedule proper meetings with structure so they are fine with not giving you a set structure. This also gives you the potential to clash with other people who just want to walk in at the same time as you. This basically puts all of the inconvenience onto you rather than on the company, which I think speaks volumes about how they view your time.
2 They are attempting to make a more relaxed, less rigid meeting structure hoping to keep you relaxed as it seems more informal. Still professional, but more informal none the less. This could be seen as a large positive as they are willing to make sure you are comfortable in their work environment.
As you can see you can get two equal and opposite views of a company doing this, good or bad, so you can't really make any assumptions about this.
I suggest treating it no differently to any other meeting, you should always be presenting yourself professionally in the best light possible.
Be sure to think about your answers and come prepared with a few questions.
Whilst it could be more informal I still suggest making sure you have some background research done on the company so you are at least partially knowledgeable about them, but honestly, I would treat it as you treat any other interview, because at the end of the day the journey is different but the goal is the same, to get you a job!
Edit: A few things probably worth taking into consideration
Don't turn up right at the end of the day, it will make you look lazy and unprepared, plus the interviewers will be tired and bored from a day of interviewing.
Don't turn up right at the beginning of the day, sure you will seem eager and ready to go, but they might still be settling themselves.
I suggest turning up an hour to an hour and a half after they 'open'. This gives them a chance to have interviewed others and got into their stride, and still gives you the chance to turn up early enough in the day to seem eager and professional.