App data is information an application stores for its operation, which can include preferences, databases, saved accounts and other persistent state. The exact contents depend on the app and platform. Clearing it can return an application toward an initial setup state, making it substantially different from a simple restart or removal of temporary cached files.
A practical example
A viewer who clears a player's stored data may need to enter the account or playlist again and recreate local preferences. That can help some corrupted-state problems, but it also removes useful information that a less disruptive step might have preserved.
A common misunderstanding
Do not clear application data without knowing how to restore the setup. Save necessary account details privately and understand which settings or local records the application may remove first.
Related terms
Further reading
Stable technical definition. Editorial approach