What is a .mp3 file?

You have an audio file. If it's from the internet, a friend, or your own music library pre-2015, there's a very good chance it's an MP3. The format has been the default way to store and share music since the late 1990s, when it simultaneously made portable music possible and broke the music industry.

MP3 exploits psychoacoustics — it removes the parts of a sound that human hearing is least sensitive to. A CD track goes from 50 MB to about 5 MB, and at 320 kbps most listeners genuinely cannot hear the difference. The tradeoff is that the data is gone forever — you can't "uncompress" an MP3 back to full quality. The patents expired in 2017, making the format completely free, and AAC technically sounds better at equivalent bitrates. But nothing else has MP3's universal support.

Every device on earth plays MP3. Every media player, every phone, every car stereo, every Bluetooth speaker. If you need higher quality, FLAC preserves everything losslessly. If you're distributing audio and want maximum compatibility with minimum hassle, MP3 at 320 kbps remains the pragmatic choice.