A streak is when you complete at least one challenge every day for 5 days. On the 5th day you will get bonus YuCoin for completing your streak.
You can see what day of a streak you are on by looking at the streak counter in the top right of your YuLife app home screen.
If you have any questions or run into any issues, please reach out to us using your in-app chat, or at support@yulife.com :-)