The app data contains all the data that has been stored since the installation, including personal data (login information) but also all content (messages, pictures/videos, documents). If the app data is cleared, all data is removed and the app is virtually reset to its origin.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Apps
Step 3:  Tap on desired  app
Step 4:  Tap on  Storage
Step 5:  Tap on  Clear data
Step 6:  Confirm with  OK
  1. Open the Settings
  2. Tap on Apps
  3. Tap on desired app
  4. Tap on Storage
  5. Tap on Clear data
  6. Confirm with OK

Samsung Instructions