exporting in mp3 format

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There is nothing on the documentation about mp3 file
can you explain bitrate?
Bitrate is the amount of bits per second in audio, and is not restricted to mp3 format, and rather is relevant on all lossy formats.
The documentation is on LMMS use and not audio formats necessarily, and if you didn't know how mp3 works, typically googling it should help you with the answer. Also, LMMS does have a note on bitrae.
The higher the bitrate, the higher amount of information pressed into each audio sample. This does NOT mean that you set mp3 export to the highest bitrate you find- often, the highest bitrate tends to be waaay higher than the best bitrate for your audio, ie you have a fair trade-off between minimum file size and maximum fidelity. Anyway, 128/160/192 kbps should generally be ok for general purposes. For when you want to provide high-quality and genuinely feel it should be needed and are willing to waste some file size space, 256 or 320 kbps should do the trick. I personally prefer 192 kbps for when using mp3 for a phone device (if I plan on using headphones or a stereo sound system, or if music is complex), 128/160 for phone devices (for typical purposes like listening from phone itself or earphones), and 320 otherwise. (I use lossless wav most of the time, a very stupid decision, but what's done is done).