Sneak is an alternative cost. If a card has Sneak, you may cast it for the Sneak cost instead of its normal cost, but only during the declare blockers step. To do so, you must also return an unblocked attacker you control to its owner's hand. The reminder text on Sneak cards spells this out: you may cast the spell for the Sneak cost if you also return an unblocked attacker during that step.
Why it matters: it turns combat into a moment of surprise. You can hold mana, let an attacker go unblocked, then return it to hand to cast a spell at a discount — or at a time your opponent didn't expect. You trade that attacker's combat damage for a trick, removal spell, or a new creature. It also lets you reuse a creature's enter-the-battlefield ability later.
