Destiny 2 is Free to Play. You can purchase it for free from any platform you play on. Some countries don't allow games rated anything other than for everyone to be Free to Play so may require a charge of a few cents, so almost Free to Play but not quite. The Seasons are content packages that change out every few months. Seasons have both free content as well as premium content available. Free content is available to everyone; premium content is available to people who purchase a Season Pass for that season. A Season Pass is to access seasonal premium content and is not required to play the game at all; it's entirely optional, just like DLC, but any unearned rewards from that content become unavailable at the end of the Season (though Bungie is going to allow Exotics to be available later on).