Short answer: Collections help you group related media and optionally present public sets on your profile.
Steps
Open Library or a media detail page.
Create a new collection or choose an existing one.
Add images or videos to the collection.
Edit the collection name, description, and visibility when needed.
Save several posts from Create
On the Create page, choose Select above your recent generations.
Select one or more posts.
Select the save icon in the selection toolbar to open Save to...
Choose a collection, or create a new collection from the picker. The selected posts are added to it.
Adding posts this way does not remove posts that were already saved in that collection.
Notes
Use clear collection names for easier discovery.
Public collections are best for curated sets, not every draft.
Troubleshooting
If a collection looks empty publicly, confirm that it contains visible public media.
If you cannot add media, refresh the page and confirm the media still exists in your library.
If the save icon is disabled on Create, select at least one generated post first.
If selected posts do not appear in a collection, refresh the collection and check that the items are completed posts you own.