Hi N4meless_King,
Welcome back to the game. :)
Seasons are tied to the dungeon objectives, which are special objectives with larger formations than normal and where DPS and CLK aren't capped at the maximum number cap.
You can do 15 objectives in the Dungeons per week, and you can buy extras if you want/need to to help with your season progress. The seasons have two reward tracks, free and extras if you pay for the season pass this season. As you earn dungeon points, you unlock the rewards on those tracks. The goal is to reach level 25 and get the achievement, but if you miss out on the main rewards, they will be available in the shop related to the dungeon when the season ends.
Runes are another source of Crusader power. Each Crusader has 5 slots for runes, though currently the 5th slot runes are being added in batches.
There's also in-game FAQs for both, and of course, community commentary. :)