Safe search removes explicit content from results and is enabled by default on DuckDuckGo. You can easily turn it off in the settings, or on a per query basis by appending !safeoff to your search (using our bang syntax). If something is filtered out as a result of safe search, we try to display a message and also offer a one-click link to disable it for that query and also a link to the settings page.
If you wanted to force safe search on (or off) you could use our URL parameters and automatically append &kp=1 (strict), &kp=-1 (moderate - don't show explicit images), or &kp=-2 (off) to all duckduckgo.com pages. You could also use safe.duckduckgo.com. Searches from safe.duckduckgo.com always have safe search enabled.
We try hard to have a pretty strict safe search by default, but sometimes unseemly things do get through. If you notice something of the sort, we'd appreciate the specific example, and then we'll do our best to resolve it in a timely fashion.