What do the icons in the "Structure" window mean?

What about (a) "f", (b) the green dot, and (c) the red triangle? And what about items with no visibility modifier?

I do not know why those extra icons/symbols are not listed on official help page – maybe worth checking Issue Tracker for existing tickets and submitting new one if none found.

If (v) is "variable" then (f) should be a "field" (as you can see it's listed under class only).

About the rest symbols/icons – sorry, no idea (I'm not PyCharm user actually).