Hi Sanjay
Virtual anchor means acting anchor not a physical anchor.
You make long line with number of symmetric loops. Check thermal axial load & displacement at the mid point support between two loop it will show zero load & displacement. Axial load become zero because of equal & opposite forces. Putting a physical anchor at that location also will not effect the result of axial load & axial movement. This point is called virtual anchor.
However physical anchor is recommended to take care of discontinuity in piping tolerance, Seismic & wind load.