Parent and child projects allow you to easily group projects and organize your work.
Child projects are nested under a top-level parent project. When you click into a parent project, you can view all the actions from the child projects nested under it.
You can also quickly switch between the parent project and children, from within the parent project, as seen here:
This organization is great for various projects or campaigns that fall under one parent project. For example, a marketing department may create a parent project called "Q3 Campaigns" as well as child projects to manage each campaign. A top level view of all campaigns for Q3 can be seen via the parent project.
How to nest a child project:
Go to project navigator
Click + New Project
Give your new project a name
Under Parent project dropdown, select the parent project