At 17th level, you gain the ability to magically steal the knowledge of how to cast a spell from another spellcaster.
Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw with its spellcasting ability modifier. The DC equals your spell save DC. On a failed save, you negate the spell's effect against you, and you steal the knowledge of the spell if it is at least 1st level and of a level you can cast (it doesn't need to be a wizard spell). For the next 8 hours, you know the spell and can cast it using your spell slots. The creature can't cast that spell until the 8 hours have passed.
Once you use this feature, you can't use it again until you finish a long rest.