Apple’s Memories feature in macOS High Sierra automatically creates curated collections of your most meaningful photos and videos. Want to save a memory for later? It’s easy.
Scroll to the bottom of the screen and tap or click Add to Favorite Memories. You'll find it in the Favorite Memories album of your Photos app. If you decide that you don't want to save a Memory anymore, tap or click Remove from Favorite Memories.
When you add a Memory to your Favorites, you preserve it and keep it from going away when Photos creates new Memories. You can also share your experiences with friends and family. On your Mac, you can share the photos and videos from your Memory when you click.
On your iPhone, iPad, or iPod touch, play a Memory movie, then tap on it to show the editing and sharing options. Tap and choose to share the movie through Messages, Mail, iCloud Photo Sharing, and your favorite social media sites.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)