Being published means that your name appears on written documents about research, and it is, indeed, important, but not essential. We’re not talking about being the first author in a publication, since this is almost impossible for an undergraduate student. However, appearing as a co-author on any presentation, publication or poster will help you build a reputation.
Keep in mind that there’s no requirement on being published, so don’t focus your energy on that, but do talk about your interest in the publication experience to your mentor or professor and they will make it easy for you. Medical schools will love to see the things that you’ve been involved with and read about what you’ve worked on!