While Milestones provide insights into a project's status and progress, Overview provides these insights across all of the active projects you're a part of.
First, access the Overview screen from your sidebar.
View all or by Portfolio
Once there, you will see the milestones of all of your active projects. Overviews provide the same in-depth view of a milestone's tasks, this module gives the added benefit of being able to filter through project collections based on the Portfolio they're a part of.