Project Date is the length of time the project will run. Envelopes cannot be sent outside of the start and end dates of the project. E.g. Black Friday is only 5 days long and you want the project to end on day 5.
Envelope Expiration: The envelope expiration can extend beyond the project expiration date. You determine how long the recipient has to open the envelope. Envelopes that are not claimed will expire.
Note: changing this period or date will not change the expiration for envelopes that have already been sent.
In the Black Friday example, you want the recipient to ultimately claim the envelope within five days to take advantage of the Black Friday sale, but extending the expiration date to December 31st will allow the recipient to make a purchase during the December holiday month.