Spell mastery is an ability that checks whether you have two or more cards in your graveyard. If you do, the spell or ability with spell mastery gets an upgraded effect—often a bonus mode, extra value, or improved outcome. It's checked when the spell resolves or the ability functions, not when it's cast.
The most common mistake is forgetting that instants and sorceries are what matter—wait, no, that's not quite it. The actual catch: spell mastery counts any two cards in your graveyard, not just instants or sorceries. People conflate it with abilities like delirium or that require specific card types. Spell mastery just needs two cards total, of any kind.
In Commander, spell mastery is easy to enable since graveyards fill quickly over long games. It rewards self-mill, fetch lands, and natural attrition, making the threshold trivial by mid-game and reliably granting the upgraded effect.