The Season pass is access to additional paid content (DLC) that will not need to be purchased separately by Season pass holders. The Season Pass is only included with the Edge of Darkness pre-order package. You can always upgrade your pre-order pack to Edge of Darkness by paying the difference between your version and Edge of Drakness.
A number of paid and free add-ons are planned for Escape from Tarkov, including new content, locations, mechanics, etc which will be released after the release of the main game.
At the moment, the Season pass already allows you to get free access to the announced Arena project when it becomes available.