There are two types of shows: episodic and serial. Serial shows allow for the concept of seasons, to distinguish common episodes numbers across different series.
When creating or editing an episode, you can select the episode number as well as the season. If the season you would like to use is not available yet, you can create a new season for the show by editing the show and setting its total number of seasons.
For example, if you are going from season 1 to season 2:
- Locate the show and click on Edit in its Overview screen
- Locate the Number of Seasons field and set it to 2
- Note the Type field and ensure that you've got the right show type selected
You will now be able to set the season field to 2 in your new episode.
The following screenshot may help locate the right field on the show Edit page.