Ideally your reference track will be a professionally released track that in your opinion 1) sounds really good, and 2) has elements / traits that you would like your song to have.

For example: If you're making an EDM track try to find an EDM song made by a professional producer that sounds similar to yours, to help give eMastered the best sense of what you're hoping to have your track sound like.

The closer the reference track is to your track in terms of style, tempo, sound design etc. the better the reference mastering will sound for your song. It's worth taking time to find great references to get the most out of the reference engine!