Todo is a syntax group , :h group-nameto check the details.
If you want to highlight a few words / text, you can either edit the syntax file or create your own hi-groups ( :h highlight-groups) and add a match.
For instance:
You can make the match todo /!Important!/text !Important!highlight the same as TODO. Or:
hi! Important ctermbg=red guibg=red
hi- Important.
:
:match Important /!Important!/
!Important! , .
vimrc.