What is the meaning of nub?

noun

• A small lump or protuberance.
  • synonyms: nubble
• A small piece.
  • usage: "a nub of coal"; "a stub of a pencil"
  • synonyms: stub
• 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, heart and soul, inwardness, marrow, meat, pith, sum, nitty-gritty