If you don't want to get scammed then don't do it...nah jk.
Banking is the action to move units into your own inbox by sending a trade to an ally or random person of your choice. You then tell them to cancel it and the units in the trade go into your inbox instead of your unit list. It's useful for keeping your unit list clean, putting away feeders, and also a way to reorganize your monster defence for your Luna and artifacts. Just make sure they're active first, so you don't get trade locked.
Use the link below and scroll to the very bottom to learn more about banking.