Mets is a dish, and entremets originally meant a little sweet dish served in between courses (although both have an ‘S’ on the end, they can be singular or plural).
In this sense, the word is a bit old fashioned, but it is now popular as another word for dessert (unless it’s a tart. A tart is a tart is a tart, I guess).
It should be something soft and mousse-y and complicated that Louis XVI might have liked. The entremets au chocolat, for instance, involves three kinds of chocolate mousse, layered on a crunchy praline base.
Image by Marianne Casamance