The Courtyard Key is a mission item which unlocks the gate beside the Wall of Light, to the east which allows access to the courtyard in Cyria Gardens. This is the area with the winding outdoor staircase leading down toward Winnow's Bar and the quarantined building. It can be looted from the guard inside the guard station by the Wall of Light. Inside the station are Squad Orders, some Pistol Bullets and a flask of S&J Health Elixir.
Note[]
This key is handy but not actually necessary. You can Far Reach / Blink to the top of the guard post and from there, to a second floor balcony off the nearest building. From there, you can access a third floor balcony, the a smaller fourth floor balcony next door which allows access to the roof of the archway over the alley. Then it is a simple matter to climb over and down into the courtyard.