Keep up the good work. Everyone who uses SW was at your learning stage at some point or another (me included!).
When you are dealing with 3D parts & features, mirroring typically involves defining some sort of plane to act as reflecting surface. It can be an XYZ origin plane or a plane which you have constructed from combinations of points & lines, or even the face of an existing part. An axis is an infinite line, so it lends itself to rotating or copying features about it. You cant 'project' an image off a line the way you can a plane.
Where mirroring is somewhat ambiguous is inside a sketch. Here you are allowed to mirror a feature about an axis. Wait, we said an axis is a line (as opposed to a plane), which appears to break the rule. But this is a special circumstance because inside a sketch you are confined to 2D space. Specifically the plane you are sketching on is fully defined & everything in that sketch occurs on this same plane. So behind the scenes, the program can solve this mirror command in the context of 2D. I guess you could say the 'equivalent plane' is known because its always the axis line projected 90-deg perpendicular to the known sketching plane.
I suspect this 'mirroring' terminology might be hangover from 2D days. And of course, we use 2D as the initial step in SW to eventually build 3D parts. But once we are in 3D space & now need to precisely make reflections or copies of 3D features, we need more rigorous definitions. I'm no expert, but this is about the best I can describe it.