Quote from Zoner's Half-Life Tools website and to be more precise, the "Command reference" section :
HLRAD -bounce #
Set number of radiosity bounces.
This option sets the number of times light bounces in the radiosity pass. By the time the code gets to this point, all the data is precomputed, and extra bounces are very fast. It will make the shadows less harsh using more bounces, but can help light up dark areas much more naturally.