On itch.io, you can add games to collections. This is helpful not only for organization, but also because the itch app will let you browse your own collections, even offline.
In the Collections tab, you can see a list of your collections, which can be filtered by a search term. Clicking on any of the collection opens it in its own tab.
Editing collections from within the app
...is not yet possible. Although, you can click the Add to collection button from the in-game browser (if the window is wide enough), and you can access the https://itch.io/my-collections page.