Question about org-emacs agenda: what does SPACE do in org-mode agenda view? It says it's for :tag-char but I can't find what it is

the spacebar?

You could try C-h k SPACE to get the command that it's calling

That's Control+h, then hit the space bar

If you want a list of all commands that are bound for your major-mode + minor-mode's, you can run C-h m

sorry it's not SPACE directly, it's when you enter / then SPACE. It's not a key in a keymap, I traced the code:

(or (eq char ?\s)
	  (setq a (rassoc char alist))
	  (and tag (setq a (cons tag nil))))

now to find out what that does :d