[count] is optional. If given, the mapping works for count lines (ie. comments out count lines).
<Leader> is usually the backslash key (\), although this can be remapped (also see What is <Leader>? and How can I find out what <Leader> is set to? And is it possible to remap <Leader>?)
cc is exactly that: two c's.
NERDComComment has nothing ...
When you register it this way, your new autocommand will be triggered after all other autocommands registered on the same event, in particular after the one that sources your ftplugin -- I suppose you define the mapping in an ftplugin (*).
You could change that, probably by changing the order between this line and the :filetype xxxx in your .vimrc, but this ...
You can use the mapcheck() function to check if the user mapped a key to something:
:echo json_encode(mapcheck('<F1>', 'n'))
:echo mapcheck('<F11>', 'n')
:set cursorcolumn!<CR>:set cursorcolumn?<CR>
So in your mapping, you can do:
if !hasmapto('<Plug>MyFunction', 'n') && mapcheck('<Leader>f', 'n') is# ''
The command is used in normal mode by typing first a count, like 10, followed by the <Leader> key (default is \) and finally cc.
Example; type 10\cc in normal mode to comment out 10 lines starting at the cursor position. Omit the count to only comment out the current line.
Many people like to remap the <Leader> key to ,. More info here: http://...
filbranden is right. Another solution would be:
^K is inserted by typing <C-V><C-K>.
A key typed after <C-V> will be inserted literally. It's very useful. See :h i_CTRL-V for more information.
A couple of points:
(Most) maps of this form that I have seen want to be run in normal mode. So you need to press your leader key followed by tn, without any colons.
Dont use regular map. It does too much, and is too easy to break. Prefer in this case a noremap variant nnoremap (for normal mode mappings).
Your command fails because vim runs the command ...
To set your leader key, you must use let mapleader = "," not let g:mapleader = ",".
From :h <leader>
To define a mapping which uses the "mapleader" variable, the special string
"<Leader>" can be used. It is replaced with the string value of "mapleader".
If "mapleader" is not set or empty, a ...