Think of closed-back headphones as a closed door, and open-back is like when you open all doors and windows to air out a room.
With an open earcup, sound waves are free to flow without being blocked. This creates a wider soundstage, richer details, and an overall impression of an open and airy sound.
However, this comes at the price of accuracy, since a wide soundstage is more difficult to achieve on headphones.