Studio Mode in OBS designed for edits during real-time events. It allows you to modify scenes in background while users don't see all of your movements with the scenes (viewers only see previously selected scene that technically named "Program").
The "Audio Mixer" pane in Studio Mode actually shows "Program" sources. It is oversight (in my opinion) that pane doesn't changes its name to "Audio Mixer (Program)" when you enabling Studio Mode.
An "Active" source in terms of OBS is the source that is present in the output i.e. in a streaming, in a recording, in a replay buffer or in a special virtual camera (there are many outputs in OBS).
If your are NOT in Studio Mode then "everything you see" right now is going to output i.e. are "active".