Meaning

• Atop

• Very close to, in any orientation or in time.

  "At peak times, a telephone operator must handle a number of calls, one on top of the next."

• In addition to something else.

  "[...] and on top of all that, I got a puncture!"

• Fully informed about, and in control of, something; up to speed with.

  "I have sorted out the problems and am now on top of the situation."