WiFi Storage allows you to run an ftp server on your android device. This means that any program can access the files on your device while the ftp server is running. For example entering 'ftp://...' in the browser url bar will allow you to browse the files on your device from a desktop pc or a laptop.
By default the user name and password are both 'ftp' you should change them. You use this username and password when accessing the server.
There is a configuration option ("keep phone awake") that fixes a problem where file transfers go slow or stop completely when the phone sleeps. If you have this problem check this box and restart the program. Your phone will stay awake as long as the server is running.