I sure can elaborate on this for you! I only just found out about it yesterday, hence why it wasn't in my official forum factions guide post, however I shall put it in there!
Faction Shield is pretty self explanatory, it is a shield that protects your claims from cannon damage and creeper damage for a duration of 2 hours. Wither's direct impact damage will still work, however. Once you type in the /f shield command, it will come up with some red glass panes, when you hover over them you will see select times of which you can pick your 2 hour time slot for your f shield to be activated. Once you select your shield, you cannot reset it, so be careful as to what time you select!.
Hope this helps.
Edit: This information is now available in my factions guide thread, which can be found here:
https://snapcraft.net/threads/24069/