This is a mechanic which allows you to replace an Aura in play with an Aura in your hand.

702.62. Aura Swap

• 702.62a. Aura swap is an activated ability of some Aura cards. "Aura swap [cost]" means "[Cost]: You may exchange this permanent with an Aura card in your hand."
• 702.62b. If either half of the exchange can't be completed, the ability has no effect.

Rules Translation

• 702.62a.- This is the rules text on the cards
• 702.62b. - This doesn't allow you to put an aura on a permanent it can't enchant, and if you try to, then nothing happens.