In Vi/Vim, ! is the portal to the underlying UNIX shell, and with it you can do powerful things. One of them is the ability to filter the lines in a range with an external command:

:'<'>! grep error
:'<'>! grep '^[A-Z]'

The first command will filter the selected lines, keeping the ones that contains the text error in them. The second one will only keep lines that begins with an upper-case character, etc.

'<'> just denotes the range of the visually-selected portion of the document. This can be any valid range, like % for the entire document.