Milestones are given each time you achieve 10 consecutive days of meditation. The 50 days milestone is awarded for every 50 days with at least one session; whether the sessions are consecutive or not.
A Yellow Star = 1 milestone
A Red Star = 5 milestones
A Green Star = 25 milestones
If milestones aren’t important to you, you can turn them off in Profile settings. (Profile > Smaller Profile > Edit Profile and Privacy > Milestones).