What is the meaning of letting down?

noun

• The act of causing something to move to a lower level.
  • synonyms: lowering

verb

• Move something or somebody to a lower position.
  • usage: "take down the vase from the shelf"
  • synonyms: lower, take down, let down, get down, bring down
• Fail to meet the hopes or expectations of.
  • usage: "Her boyfriend let her down when he did not propose marriage"
  • synonyms: disappoint, let down