Flashcards is an app to create and study flash cards. It works in the browser as an offline app. The app is very simple. For more advanced use cases, see Wikipedia: List of flash card software for good solutions.
The app lets you create flash cards with a front (text) and back (text). In a study session, you review each card and note whether you remembered the answer or not. Cards which you remember will appear less frequently while cards you failed to remember appear sooner. This method is called spaced repitition.
The app works offline. Open the app in your browser and select "Add to home screen". This adds an icon to your home screen and makes the app available offline.