The first time you visit most major pages — Dashboard, Games, Mall, Achievements, Ranks, Chat, Weekly Missions, Movies, Profile, Gift, and a few others — you'll see a short welcome popup with a quick explanation of what that page does and how to get the most out of it.
These popups are meant to save you from having to guess what a feature does, and each one only appears once. After you dismiss it, Chillverse remembers that you've seen it and won't show it again on that page.
If you skipped through one too quickly and want a refresher, your best bet is checking the relevant Help Center category for that feature (for example, the Games or Diamonds & Mall sections), since the popup itself won't reappear automatically.