A milestone is an expectation for a meeting or deadline for students to keep the project on track.  You can see  milestones that have been completed or milestones that are past due by clicking on the "Teams" tab on your project page and checking out the progress bar.

When students complete a milestone, you will be alerted with a notification and this will be updated on their milestones progress bar. You can see the submission notes and download the deliverable (if there is one) by clicking on the team, then the name of the milestone as shown below. 

https://cdn.elev.io/file/uploads/XsLpRj5zxXer9xeIiXQYh8YNzGG-SbiTvHn2Op8LrxY/OxCGeVaCkT1ZZFtmryNzQXdg88voiVryhiDBl366xgs/Check Milestones_-Wjs.gif

After you're done reviewing the milestone, click "Approve" to confirm that the milestone has been completed by the students. Reject the milestone if the students have not completed the milestone adequately.

As the project progresses, you can review milestones by clicking on the team name.