C-u C-SPC is not bound to a single key sequence.
C-u is bound to
C-SPC is bound to
set-mark-command. If you consult the doc for each of those you will get the answer to your question.
C-u provides a prefix argument for the following command, in this case for command
set-mark-command. Consulting the doc for
set-mark-command tells you how it interprets a prefix argument:
With prefix argument (e.g., C-u C-SPC), jump to the mark, and set the mark from
position popped off the local mark ring (this does not affect the global
mark ring). Use C-x C-SPC to jump to a mark popped off the global
mark ring (see
set-mark-command-repeat-pop is non-nil, repeating
the C-SPC command with no prefix argument pops the next position
off the local (or global) mark ring and jumps there.
With C-u C-u as prefix
argument, unconditionally set mark where point is, even if
set-mark-command-repeat-pop is non-nil.
Consult the Emacs manual for more info about prefix arguments:
C-h r i prefix arguments RET. That takes you to node Numeric Arguments of the manual.