chmod is a Linux/Unix shell command which is used to change permissions of files and directories. chmod is an abbreviation of change mode. Under some web hosting environments, a user may need to use this command to change some file or directory permissions to make them writable by WordPress. If users don’t have access to a Unix shell, then they can still use this command by using an FTP program.
In plain English: there are times when you want to add a plugin from your WordPress dashboard, upgrade a plugin, add image, or something similar. In some of these cases based on your web hosting settings, you might not be able to do any of that because the file/folder permissions are not correct. In this case, you would need to use the chmod.