Sub addresses work the essentially the same as your primary vault address but have a couple of useful functions.
You can set up separate sub addresses for individual payments that you receive to an account. They are unlinkable so therefore do not compromise user privacy. They can be named for receiving different payment types to a single vault. You could use them for mining rewards or receiving invoice payments. All funds received into sub addresses can be psent by using the primary address of a Haven account.
Using sub addresses have the advantage of being able to manage multiple streams while only requiring the use (and recording) of a single seed phrase.
IMPORTANT NOTE: Whilst subaddresses are useful for separating out payments recieved to an account it is not possible to send funds from a specific sub address. You can think of the separation as a soft grouping.