argv0 realname zero argsrealname is one argument. zero is one argument. args is any number of arguments.
argv0 runs the program stored as realname on disk, with zero as the 0th argument (rather than realname) and args as the remaining arguments.
argv0 /bin/csh -cshruns /bin/csh with a 0th argument of -csh. /bin/csh will think it is a login shell and behave accordingly.
As another example, the command
tcpserver 0 ftp argv0 tcpd ftpd -l -Ahas a similar effect to the line
ftp stream tcp nowait root tcpd ftpd -l -Ain /etc/inetd.conf. The tcpd program is run with 0th argument ftpd and remaining arguments -l -A. Note that tcpd can and should be replaced by the -x option of tcpserver:
tcpserver -x ftp.tcp 0 ftp ftpd -l -A