It’s not a sign that they’re considering you–they try to interview everyone.
The interview is not super important. They will for sure ask about why you want to go to Dartmouth, so be specific in your answer. Know how to talk about yourself–what academic subjects interest you, what you put as your intended major and why, what activities you have participated in during high school, and your impact on them. Prepare some thoughtful questions–make sure the answers to these questions can’t be easily found on the Dartmouth website. All in all, don’t stress too much about the interview; it is just a conversation. Dress nice, arrive on time, and be polite. Relax and smile! Most people have good interviews, so positive interviews don’t typically have a huge impact on decisions, but another small boost helps. I think negative interviews can impact you, but the interview would have to be really bad, so don’t stress out about it! And also don’t read too much into what the interviewer tells you. I had a really fantastic interview for Brown ED and my interviewer said I would be perfect for Brown, and he doesn’t see why they wouldn’t accept me. He said he would write me an amazing report and guess what? I was deferred. So the interview matters very little in the process.
Also I suggest maybe looking up your interviewer, and seeing where they work, etc. Maybe you can tailor some of the things you say to their interests. I couldn’t find much about my interviewer, but I did see that he went to law school on his LinkedIn and he studied history at Brown so I made sure to talk about my interests in government, history, and politics.
Good luck!