This is a tiny question, but I am a stickler for understanding the details of any program I use. Using the default startup scene in Blender, for example, if you select the cube only, it is the active object. In the lower left corner of the 3D View window, the text “(1) Cube” is showing this fact. I can’t find any documentation to explain, but what does the “(1)” part of that text mean?
Frame number 
Wow! Thanks! I can’t believe such a little thing has been bugging me so much, maybe because I still haven’t found where this is documented. Whew!