Your iPhone will scan the pictures in your photo library and try to identify what's in them to help you search

To access it, just hit the search tab bottom right and type in what you're looking for

You can search for simple things like 'Beach' - when you see a suggestion that matches under your search just press it

You can search for more advanced things like 'Sunset Sunrise' pictures

If you have people you know already added, you can search for them by name

And combine those searches with objects - like sunglasses

You can combine people, to find photos with both people in them

Or search for Disguise to show costumes and combine that with Holiday

Or combine searches like Dog with Snow to find a particular photo