What's the meaning of 'inoremap' in vimrc

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

For more on why the command has such a bizarre name see this excellent description between the difference between map and noremap. Really good to know!

To summarise that article, here's a choice quote:

One downside of the *map commands is the danger of recursing...

Vim offers another set of mapping commands that will not take mappings into account when they perform their actions.

So noremap came about to avoid horrible recursion of mappings like

:nmap dd O<esc>jddk 

where the dd in the right-hand side of the map recurses back to the left-hand side definition of the map, and Vim gets stuck in an infinite loop!

Answer 2

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.