Definition
PlayerPrefs saves strings, integers, and floats under a key. Unity stores them where the platform keeps preferences: the registry on Windows, a plist on macOS, a shared preferences XML on Android. The API is one line to read or write, which is why it is often used beyond settings for coins, unlocks, and progress.
Relevance
PlayerPrefs is the right place for a volume slider or a last-used name. The values are plain on disk, so it is a weak place for coins, unlocks, or anything the player should not be able to edit with a text editor.
Example
The game saves the music volume with PlayerPrefs.SetFloat("music", 0.8f). On Android that number is in an XML file under shared_prefs. The next launch reads the same key back.