What is the meaning of heart and soul?

noun

• The choicest or most essential or most vital part of some idea or experience.
  • usage: "the gist of the prosecutor's argument"; "the heart and soul of the Republican Party"; "the nub of the story"
  • synonyms: kernel, substance, core, center, centre, essence, gist, heart, inwardness, marrow, meat, nub, pith, sum, nitty-gritty

adverb

• With complete faith.
  • usage: "she was with him heart and soul"
  • synonyms: body and soul