boil down

1.  Simplify, summarize, or shorten, as in John finally managed to boil his thesis down to 200 pages.
2.  boil down to. Be reducible to basic elements, be equivalent to. For example, What this issue boils down to is that the council doesn't want to spend more money. These metaphoric usages allude to reducing and concentrating a substance by boiling off liquid. [Late 1800s]

The verb has 3 meanings:

Meaning #1: be the essential element
  Synonyms: reduce, come down

Meaning #2: be cooked until very little is left; as of sauces
  Synonyms: reduce, decoct, concentrate

Meaning #3: cook until very little liquid is left, as of sauces
  Synonyms: reduce, concentrate