Your teams are organized for play during a specific period of time or season. The teams that you'll define later exist only for that period of time. Typically, a team might play during the spring or fall, or perhaps for a whole year. The "Season" form allows you to define a season's name as well as it's beginning and end dates. Once defined, you'll use it to assign teams to and thus define their playing season.
To create a season, you'll use the "Seasons" menu choice on the Admin menu, Admin > Teams > Seasons. When clicked, this choice brings up a Season listing page that displays the seasons already defined. At the top of the page is a link titled "Add new Season." Click this link to bring up the Season Information form. The form has the following fields:
Type the name of the new season here, up to 32 characters. This is the name that will be displayed to the public so it should be descriptive but also as short as possible, such as "Spring 2012."
Keep in mind, you can only delete a season if all of the teams and leagues associated with that season have been removed.
Also, there is no reason to delete a season, unless it was a mistake :-). You can however hide seasons by editing each teams' settings and making them Not Public.