thisComp.layer (Medium Gray - Royal Blue Solid 1"). transform.opacity
which means that within this composition there is a layer named MediumGray - Royal Blue Solid 1 and the opacity property is being modified within theTransform menu.
Some properties like position and scale have two values. A list of values is referred to as an array. For example [position[xvalue],y value] is an array of the xposition and the y position values. The scale property is a one dimensional array. If we take the pickwhip and drag it to opacity, which is a one dimensional value, then in the expressions bar a variable and a two dimensional array will be created [temp,temp].