When a variable is set to Editable, this allows the variable to be modified from outside of the Blueprint that the variable resides in.
You can set a variable to Editable from either the Details panel under the Editable setting, or by clicking the Closed Eye icon next to the variable inside the MyBlueprint window. When the eye icon is closed, the variable is currently set to non-editable (which is the default setting). When the eye icon is open, the variable is set to Public (or Editable as seen below).
When a variable is set to Editable, you can set the value of the variable from the Details panel of the Main Editor window.
Above, the variable LightColor has been set to Editable and we can now set this value in the Details panel of the Level Editor.