What is the meaning of boil down?

verb

• Be the essential element.
  • usage: "The proposal boils down to a compromise"
  • synonyms: reduce, come down
• Be cooked until very little liquid is left.
  • usage: "The sauce should reduce to one cup"
  • synonyms: reduce, decoct, concentrate
• Cook until very little liquid is left.
  • usage: "The cook reduced the sauce by boiling it for a long time"
  • synonyms: reduce, concentrate