It's sort of a meta search engine that utilizes other search indexes. Here's a post from the founder.

https://duck.co/forum/comment/27893

Edit, that's about 5 years old. The current page on Wikipedia provides a more recent explanation.

"DuckDuckGo's results are a compilation of "about 50" sources,[42] including Yahoo! Search BOSS; Wikipedia; Wolfram Alpha; Bing; its own Web crawler, the DuckDuckBot; and others."

https://en.wikipedia.org/wiki/DuckDuckGo#Overview