CHMOD means to change the permissions on a file to let others view it, change it etc.... Normally, webpages are set up so only the webmaster can change them, but the forum requires permissions set to let user activity update certain files (register, add posts, edit profile etc..).
You use your FTP client to CHMOD.
I use WS_FTP. In this client you right click on the uploaded file (on the website) to be CHMODed, select FTP commands, then SITE, then enter the file to be changed & CHMOD permisisons as stated in the Invisionboard instructions: like this: CHMOD conf_global.php 0755 .Then do the next file.