Disturb lets you cast certain double-faced cards (usually creatures) from your graveyard, but you cast the back face—typically a cheaper, often Aura or spirit version. After casting it this way, if it would leave the battlefield or the spell leaves the stack, you exile it instead of returning it to the graveyard. So Disturb gives you a one-time second use of a card.
The most common mistake: people forget the exile replacement only applies when you cast it with Disturb from the graveyard. If the card dies normally from the front face, it goes to the graveyard like usual, and you can still Disturb it later. Also, you're casting a spell, so it can be countered, and Aura-based backs need a legal target.
In Commander, Disturb adds resilient card advantage and graveyard value, dodging single-target removal by giving you two threats from one card—great in attrition-heavy or grindy decks.