What is Season Pass and How Does it Work?

Hello guys! As the title says, I was wondering what is a Season Pass, how does it work, and what are its benefits?

I'm a relatively new PS4 owner and about two months ago, I purchased the Nioh Season Pass, which back then contains the Dragon of the North and other new content.

I also want to get the Defiant Honor and the upcoming Bloodshed's End. The PS4 store says that they are free if you have a season pass. I'm a bit confused if that Season Pass is the same one that I bought.