Every file on a Unix-based filesystem has three levels of access permission—read, write, and execute. These permissions are assigned to three separate entities—the owner, a user group, and the world. Best practice dictates that these permissions be set as low (restrictive) as possible (that is, unless you really NEED world-writeable files, don't set them that way!).
It's important to ensure that the filesystem permissions for all of the phpList's files are correct. If the permissions are too restrictive, the web server won't be able to display the phpList interface at all. If they are too loose, you risk unauthorized users editing or deleting your files.
If you unpacked phpList onto your web host directly from the source tar.gz file, then the permissions will be correctly set. However, if you transferred the files from another host using FTP, it's possible that the permissions will have been reset. Regardless, it's advisable to inspect these.