Collections is a place to keep all your bookmarked icons.
Go to your collections by clicking on the bookmark icon on the top-right corner.
Your collections page will look like this:
Collections are a good place for organising your bookmarked icons by project, theme, style or whatever sorting makes sense to you. It makes it easier to find the icons later on and to share them with your colleagues.
Adding an icon to a collection is as easy as clicking on the little bookmark button:
Or by clicking on the "Add to collection" button on the icon page, top-right.