There can be confusion between Label and Folders, generally. Gmail uses 'Labels' and some of the other IMAP service providers have 'Folders' but they act very similar to Labels. See below that how Airmail works:
For Gmail/Google Apps Accounts:
- Folders are called labels because Google allows a message to belong to multiple labels at the same moment. It simply means that 'A single copy of a message' can be accessed through all the labels which the message has on it.
For POP3/Generic IMAP/Exchange Accounts:
- Folders are used and a single message has to be copied to all the folders if you wish them to remain in multiple folders at the same point of time.