Think of it like seasons of a TV series, with the different DLCs being episodes. So the Season Pass 1 gives you the three DLCs that came out in 2019, Season has three DLCs from 2020, and the Season 3 Pass will do the same with the three DLCs coming in 2021. That does not mean that they are time-limited, but that you can get the DLCs released during that Season at a lower price.
For Season 1 and 2, you can each save 10€ by buying the Pass versus stand-alone, plus you get some exclusive cosmetic content. We haven't announced pricing for Season 3 yet, but you can expect a similar approach to offering some savings, and some exclusive cosmetic content.