It means that after downloading license you have 72 hours for activating your software with the downloaded key. If you won't make it within that time, you'll have to log-in to your User Area account and re-download the license key one more time, because key looses its validity after exactly 72 hours after download.
This also implies you can't store such a keyfile somewhere on your disk forever (e.g. on an event of system crash and re-installing).