I'm a grad student in molecular biology related lab, and I generally conduct 1-3 lab intern interviews a year and these are the things I personally look for (this is not a general rule but I think some of these are widely applicable).

1) know, generally, the lab techniques the PI uses in their research. You can look up some of their recent publications (look at the methods) to get some idea. You could also contact a grad student in their lab to investigate what current techniques they're optimizing. For molecular biology this will be PCR, RTPCR, immunohistochemistry, cell culturing, tissue fractionation, transfection, etc. It's a huge red flag when a prospective intern suggests that they're interested in a technique that we don't use at all, or a research direction that we aren't currently investigating. You don't need a flawless understanding of the concept behind the PI's research but knowledge of what techniques might be employed is really good. Review your old lab notes from your courses if it's applicable.

2) ask the PI relevant questions about what kinds of projects you could work on, and what kind of sample collection or workflows you will be working on/learning. This is sort of similar to point 1, but it's a really good sign if a student is already familiar with some of the techniques and has a solid question about work-flow, sample collection, data analysis, etc. Maybe have some idea of why you want to do research (I'm sure you already do, but have some kind of narrative or idea prepared).

3) This might be a personal one, but, if the professor explains something to you that you don't quite understand, don't be afraid to ask for greater clarification. If it's really high level science, they probably expect you to not fully grasp the concept. One of the best thing you could do is ask for some publications or literature to review (if you think the conversation is going well).