The header that identifies the file as an extended M3U.
The .m3u extension stands for "MP3 URL" or "Moving Picture Experts Group Audio Layer 3 Uniform Resource Locator." While originally for audio, it is now the standard for IPTV (Internet Protocol Television) playlists.
Since these files contain external links, always use a VPN to protect your privacy and ensure your device has up-to-date security software to block potentially malicious stream redirects.