SafeSearch is a feature that allows you to control what turns up in your searches on Flickr, and it's on for everyone by default.
Levels of SafeSearch
If you're not signed in to Flickr, SafeSearch is always on.
If you're signed in and under 18, you can select SafeSearch on or moderate.
If you're signed in and over 18, you can select SafeSearch on, off, or at a "moderate" setting.
You can set a default SafeSearch level that you'd like to use (if you're signed in). You can also change this setting per search via Advanced Search page. SafeSearch filters also apply when you're browsing around.