What does it mean to have mild c4-5 and c5-6 posterior disc bulge and anterior thecal sac indentation?

Disc "BULGES" are NORMAL findings. ALL HEALTHY discs BULGE when the vertebrae MOVE since they are FILLED WITH FLUID thus acting as the "cushions" between the vertebrae. When they bulge towards the spinal cor they may "indent" the SAC in which the cord "floats".
Thus the report you refer to is a NORMAL finding!
Hope this is helpful
Dr Z.