This indicates a possible attempt to exploit a vulnerability in some versions of the AWStats tool.
AWStats is a free tool for generating graphical statistics from web, mail and ftp server logs.
A security hole exists because of insufficient input validation when AWStats is used as a CGI program. A remote user can execute arbitrary commands starting with | on the web server, with web server's user permissions.
AWStats 6.95 and older.
Attacker can execute arbitrary commands using web server user permission.
Apply the patch supplied by the vendor: