When you fist start using your HTC One M8, some quick tips are shown (Green pop-up balloons) to help you get the most of your device. To disable or review all tips again, do as follows:
1. Open Settings app and go to About.
2. Open Help.
3. To disable showing the tips, make sure to unmark the "Show all quick tips" option
4. If you want to see all the tips again, touch "Review all tips again" and confirm the process.