Hello, Rita. This is an idiom and a phrasal verb. If something boils down to something else, that something else is the reason for it, the essence of it, or the most important or most basic part of it. Imagine boiling a pot of some liquid until nothing is left except some residue; that is the essence of whatever you are boiling. This is the origin of this idiom. So, in your example, the sentence means, "Dinner is okay, but dessert is always the most important (or best) part of dinner for me."
An example is: He seemed like a nice guy and I enjoyed our first date, but, for me, it all boiled down to whether I felt a genuine connection with him.
Hope that helps!