An M3U link is a playlist address that a compatible IPTV player can read. It is not an app by itself and does not guarantee quality or authorisation.
What to check first
When entering M3U details, copy the full URL carefully. Missing characters or expired links are common reasons for empty categories.
Practical Firestick advice
Keep playlist details private, and use sources only where you have the right to access the content.
Use IPTV apps and playlists only with sources you are authorised to access. A player app does not provide channels by itself.