Collections are a simple way to bookmark and organize the things you find on Product Hunt.
To create a new collection or add something to a current collection, click the ribbon with the plus sign when viewing a product under the "Follow" button.
You can then either create a new collection with that product or add the product to an existing collection.
You can also work on collections with others by adding them as a collection curator. You can read more about curators here.
To see all of your collections, simply visit your Product Hunt profile.