It's C-u <Space> in Emacs lingo; it pops the mark ring. Some actions like C-<Space>, M-< and M-> push the position of the point to the mark ring before moving it, and when you pop the mark ring, you jump to the position that was pushed most recently.

> Some actions [...] push the position of the point to the mark ring before moving it

Notably C-s and C-r (incremental search forward/backward). That in combination with C-space is the way to navigate in emacs like a pro :-)

(btw: its C-u C-space. C-u space inserts 4 whitespace)

TIL! I didn't know search pushed marks, thanks!