Apple revamped the Photos App in macOS Sierra, adding a Memories feature, which is designed to help you “rediscover favorite and forgotten moments from deep in your photo library.” Memories automatically creates curated collections of your most meaningful photos and videos.
To use it, open the Photos app and click Memories in the left sidebar. You'll see a summary of the Memory's contents, chosen from the best photos in your library. You can choose Show All to see every photo and video in the Memory.
Scroll down to see the people included in the Memory, a map showing where the photos and videos were taken, and a list of related Memories for you to explore. At the bottom of your Memory, you can find options to add the Memory to your favorites or delete it.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)