What is the meaning of drop off?

verb

• Fall or diminish.
  • usage: "The number of students in this course dropped off after the first test"
• Change from a waking to a sleeping state.
  • usage: "he always falls asleep during lectures"
  • synonyms: fall asleep, dope off, flake out, drift off, nod off, doze off, drowse off
• Leave or unload.
  • usage: "unload the cargo"; "drop off the passengers at the hotel"
  • synonyms: drop, set down, put down, unload, discharge
• Retreat.
  • synonyms: fall back, lose, fall behind, recede
• Get worse.
  • usage: "My grades are slipping"
  • synonyms: slip, drop away, fall away