For reference, see
http://www.ss64.com/nt/xcalcs.html.
The XCACLS is the name of the program file that gets executed when this command is used, H:\*.EXE refers to all executable files on drive H, the parameter /T stands for "traverse all subfolders and change all matching files found", the parameter /C stands for "continue on access denied errors", the parameter /D followed by %USERNAME% means to deny specified user access and this will over-ride all other permissions the user has, and the parameter /Y stands for "replace user's rights without verify".
The lines after that have the same effect but for other file types.
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