Adding a maintenance period for a license gives you an additional timestamp to work with than the validity period/expiry date of the license.
For example, if you wish to issue a license that never expires, but you wanted to charge for updates, you would set the license type to perpetual and just set a maintenance period expiry date.
Go to the license detail page (or "add product page" if you are issuing a new license), and check the box "enable maintenance period".
You can select "maintenance duration" if you would like to set an end date once the license has been activated, or you can specify the end date with a calendar day.
If you set a "maintenance duration" on an already activated license, the count-down will begin using the current date. If the license is not yet activated, the maintenance will will only start counting once the license has been activated.
You can also add a maintenance window on a license policy, but given that the license is not being issued, LicenseSpring would not know when to set an end date, and therefore only a "maintenance duration" can be set.