:help CTRL-C
:help v_CTRL-C
:help c_CTRL-C
:help i_CTRL-C
Interrupt whatever Vim is doing and/or put Vim back in normal mode.
There is actually not much to say about it. It acts like the <ESC>
key in most cases.
You can use it to quit a commandline call without executing it or interrupt long running processes.
It also ends insert, replace, visual and select mode.