Vim allows you to filter lines from your current buffer through an external program. For instance, if you have some ugly looking json that you’d like to format in a readable way, you might want to filter it through an external json pretty printer program.

To filter the entire file through an external program, use

:%!! <external-program>

Or you can make a visual selection and just filter that

:'<,'>!! <external-program>

See :h !! for more details.