Seasons are periods of time with new content additions, changes to game balance and the reset of competitive leaderboards. It is also a fresh start where everyone making a character at the start of a new Season will start from scratch.
While Season is active, you can choose to create a character in the current Season, or in Legacy. All Seasonal characters will transfer to Legacy at the end of a Season and all progress will carry over and will not be deleted.
During a Season, some features may only be available in the Seasonal environment. An example of that could be Season-restricted items.
If dying with a character that is playing Hardcore Game Mode in a Season, that character will not become a Normal Legacy character, but instead Normal Season character.