A lot of instrument aproach charts (VOR or ILS) have a circle around the navaid. It can be 10/20 NM or in km, like the 20km circle in the image. What does this circle mean?
It can be 20 km, or 10 NM.
Since you've shown an ICAO-style plate, ICAO Annex 4 Aeronautical Charts says:
18.104.22.168 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.