Medicine is a team sport. To gauge your collaboration skills, some schools will assess you in teamwork activities that you complete with another applicant. The pair of you might be asked to create an action plan together or one of you may act as an instructor and the other as a doer. (And then that gets flipped in a second exercise.)
An interviewer might ask you to build something with Lego blocks or do a puzzle together within a short timeframe. Only one of you can see a picture of the finished product, and the other person must listen to verbal instructions to assemble it. Medicine is intensely collaborative, so as silly as it seems to build a primary-colored train together, this activity will show some important communication skills. (And because more medical schools are shifting towards small group learning experiences—they want to see how you'd behave in them.)
Other schools may ask you and a larger team to find a solution to a problem or create a plan for a future program together. Things can get a little more heated here because opinions play more of a role. The most important thing to remember is that your teammates are not your competition, they are your colleagues. Listening to and empowering your teammates is a big deal. If you notice someone being left out of the conversation, be the person who asks, "What do you think, Tony?” If you're blocking out what a teammate is saying because you're waiting for your turn to speak, you will not do well in this team activity.
What if I don't finish?
That's okay! This isn't really about building the Lego train. You might be paired with a complete dud of a partner, someone who is so nervous that they can't focus. If you're the instructor in this activity, keep your cool and show patience with your partner. Search for new ways of explaining the same concept. If you are the doer, ask thoughtful questions. This is going to reflect well on you, finished project or not.
Related:
The Med School’s MMI Interview: Moral & Ethical Situations
The Med School’s MMI Interview: Healthcare Policy Questions
The Med School’s MMI Interview: Role Playing Scenarios
The Med School’s MMI Interview: Writing and Picture Stations