To open the Global Menu click the Menu button in the upper left corner:

Global Menu is storage for your collections, projects, and boards. 

To add boards to a collection, click the Settings button and choose the necessary boards. 

There are default collections:

Favorite Boards collection is displayed when at least one board is marked as favorite. To mark your boards, click on the "star" icon near the board name(in the Global Menu or the board itself). 

Recent Boards collection displays a list of your recently opened boards.

Boards collection contains the boards without any project or collection assigned.

Near the Global Menu icon and Search bar you may notice + icon. Choose it to create a new board, collection or project.