What is the meaning of one by one?

adverb

• In single file.
  • usage: "the prisoners came out one by one"
  • synonyms: one after another, one at a time
• One piece at a time.
  • usage: "she sold the plates by the piece"
  • synonyms: by the piece
• Apart from others.
  • usage: "taken individually, the rooms were, in fact, square"; "the fine points are treated singly"
  • synonyms: individually, separately, singly, severally, on an individual basis