Drag & drop boards into one another to add them as sub-boards, just like how you organise folders on your desktop. 

You can drag a board from the dashboard and drop it into a board in the sidebar. Click the '+' icon to see what sub-boards are nestled within the parent board. 

Or, you can drag & drop boards into one another from the dashboard and click the '...' icon to see it's sub-board list.

If you find it easier, you can just drag & drop to rearrange them from the sidebar.