They're just quick redirects to specific sites' searches, which work wherever DuckDuckGo would otherwise return search results. They make it really quick to search specific sites, or even to go directly to specific pages on some sites (without remembering the URL's exact syntax).

Try searching for “!w search engine”: DDG sends you straight to Wikipedia, which sends you directly to the relevant article. (Search “!bang” for more info about bangs.) When DuckDuckGo is your default search engine you can do this straight from your browser's address bar or OS's search field. It's very nice.

(I have no affiliation with DuckDuckGo; I just really like this feature.)