it means that if your ftp server for your site is a Unix server (unix, linux, bsd, any nix in general) that you need to CHMOD (which means to change permissions) the files to 755 (which will allow the webserver to "serve" them to people browsing your site). in general if you right click on a file in your ftp server, it will give you a list of commands, one should be "Change permissions" or "CHMOD" or something of the sort. clicking on it will then give you the options of what permissions you want to set on it. hope that helps.
