The main goal for each rotation should be to get a sense of how well you fit with your professor and your labmates. Having a good relationship with your professor and labmates is one of the most critical factors to your success in your PhD program.
It can be helpful to think about how the lab’s research directions line up with your career goals. Specifically, consider whether the skills you will be learning in this lab will be helpful for finding a future job in academia/industry and look into what types of jobs that the lab alumni have moved on to.
Finally, while it is important that you are interested in the research performed in the lab, mentorship/personality fit is often more important to grad student success. If you do not get along with your advisor, the PhD/MS will be a difficult process no matter how interesting the research is. On the other hand, if you get along well with your advisor, you can almost always work together to develop a research project that you’re interested in.
If you are an MS thesis student thinking of transitioning into the PhD, please look at the MS to PhD Transition guide for more tips. You might feel more pressure to be productive during your rotation to convince a PI to fund your transition into the PhD.