Seasonal billing allows you to bill and entitle during a specific season and not at other times of the year. When a Season Set is attached to a billing plan the billing will only occur during the seasons within that Season Set. Entitlements by default will also be for the season although you can extend them before and after the season or have them on continuously for the whole year.
A Season Set contains a list of seasons which allows you to renew for the next season simply by adding another season to the set.
Example: If you would like to bill for the football season from September to December on a monthly basis you would create a billing plan with a season set containing the season from September to December of the year in question. If you want to extend entitlements to include the pre-season and post-season you could do so by starting the entitlements a month (or more) before the season and ending them a month (or more) after the season.