What does a 10nm/20nm circle on instrument approach charts mean?
It can be 20 km, or 10 NM.

Since you've shown an ICAO-style plate, ICAO Annex 4 Aeronautical Charts says:


  11.3.3.1 Except where this is not practicable, a distance
  circle with a radius of 20 km (10 NM) centred on a DME
  located on or close to the aerodrome, or on the aerodrome
  reference point where no suitable DME is available, shall be
  shown; its radius shall be indicated on the circumference.


So whenever possible, this circle must be shown.