What does the symbol and rating mean on this power plug?
The no-load power input is the power the supply itself consumes when is not delivering power to a load.
The supply DC output rating is 22 V 0.75 A, or 16.5 Watts.  The 100 - 240V 0.5A rating is probably a minimum rating required by some electrical codes.  The supply should not draw anything near that if it only delivers 16.5 Watts.