After the colonists successfully resisted the Stamp Act enough to get it repealed, Parliament thought it needed to save a little face. Their solution? The Declaratory Act.

Basically, the Declaratory Act said Parliament had the right to make laws like the Stamp Act. They didn't make any at the time, and had just repealed the Stamp Act because of colonial pressure, but sure, they had the right to.

This is about as convincing as it sounds—like a dog who runs away from you only to turn around and savagely bark when there's a door between you. Passing the Declaratory Act was an admission that Parliament was toothless, and the colonists knew it.