It depends who you ask.
The commonly held belief is that it is the advancement of the tech track ladder. So that you can have more responsibility but without becoming a [people] manager. So in this pretend world, staff plus engineers can dictate technical and architectural direction, and perhaps corral resources, of the company without being a manager. You are expected to make decisions that can affect everyone "under" you. For example, you might dictate (either by coercion or persuasion, depends on the company as much as your own style) migration to React or some other crap, from whatever current crap you are using. Or decide code style, and write or cause to be written, enforcement tools for such.
But more cynically, and as I believe, it's a way to pay long time employees, that hold critical tribal knowledge, more money. The title has become pretty disconnected from authority/responsibility, even at Google.
If you search for [principal engineer], you'll find some different thoughts on staff plus leveling.
It's quite common terminology, so I'm pretty surprised after 20 years you'd have no idea. You must be in a pretty insulated bubble.