Blitzball is one of the biggest minigames in Final Fantasy X. In essence, it is basically underwater soccer, where the objective is to score more goals than your opponent. You get to play one game during the main story (you don’t need to win it), but after that, it is completely optional and accessible from any Save Sphere. If you plan on doing everything in the game, then this will come naturally, as Wakka’s Overdrives and Jupiter Sigil are tied to Blitzball.