I don't why they use "pre", but that's how the device OS (separate from iFly) indicates that it remembers the credentials for a particular network. Networks that have not been logged into before (or have had the credentials deleted since the last login) do not have the "pre" indication.
Even though it doesn't seem like it should matter, those saved credentials can somehow cause problems logging into other networks, so it's best to try to avoid them (or to only have one "pre" network--the one you always connect to inside your plane). I don't have a 740 and can't give you the exact path to follow to delete the credentials and remove the "pre" indication.
Attempting to log into a wifi network at home (or in a hotel) for internet access and then log into the ADSB wifi inside the plane is a classic case of when the "pre" networks cause problems. Either the remembered credentials from the plane mess up connecting to the home wifi, or the remembered credentials from the home wifi mess up the connection to the ADSB network.
It's really best to just avoid that hassle by only ever connecting to the network in the plane.
There's little to be gained by connecting the 740 to other networks anyway. For general internet access, other devices like a phone, tablet, or laptop will give a much more satisfactory experience, and these days almost everyone has access via those devices (or can pick up a device for pretty cheap--Android tablets suitable for general web browsing can be had for $100, or even less). For iFly update purposes, it's faster/easier to remove the iFly SD card and use iFly Connect on a Windows PC/laptop. If that's not practical for you, then generating an update package on a USB thumb drive is stil usually a better option than fighting the wifi headaches.