What are you trying to do? It looks like you are trying to convert commas to newlines, in which case it's
which means from line 1 to last line ($), substitute comma with Ctrl-V Ctrl-M (ie a new line). Do not put the space between ctrl-v ctrl-m, that's just to make it easier for you to read. The 'g' on the end means all occurrences on each line. By default it would only match/amend the first occurrence on each line.
The vim docs are at http://www.vim.org/
and are also on your system somewhere eg /usr/share/vim/vim62/doc (62 = version 6.2 : YMMV).