Question

There is a line below in vimrc example file

inoremap Ctrl-u Ctrl-G u Ctrl-u

What’s the meaning of inoremap and what’s the function of this line?

Answer 1

The vim :help inoremap is very poetic about this:

:ino[remap] {lhs} {rhs}         mapmode-i             :ino :inoremap
:ln[oremap] {lhs} {rhs}         mapmode-l             :ln  :lnoremap
:cno[remap] {lhs} {rhs}         mapmode-c             :cno :cnoremap
                    Map the key sequence {lhs} to {rhs} for the modes
                    where the map command applies.  Disallow mapping of
                    {rhs}, to avoid nested and recursive mappings.  Often
                    used to redefine a command.  {not in Vi}

Thus it makes some insert-mode mappings for ^U that show the filename (^G, undo the most recent change (u), and scrolls the buffer upwards by half a screen (^U).

I have no idea why someone would want this specific sequence of commands, except to demonstrate the inoremap feature — the ^U at the refers to the meaning the command had when the definition was created, rather than calling back into the redefined ^U mapping.