Audio compression reduces the amount of data required to store or transmit sound. The right setting depends on the source, the destination and whether the file will be edited again.
Lossless and lossy compression
A lossless codec such as FLAC reconstructs the encoded signal exactly. It is useful for archives and working copies.
A lossy codec such as MP3, AAC or Opus discards information according to a psychoacoustic model. This can dramatically reduce file size, but the discarded information cannot be restored later.
Converting a low-quality MP3 to FLAC therefore does not recreate the original recording. It only stores the already degraded signal without introducing another lossless-stage change.
What bitrate means
Bitrate expresses how much data is allocated per second. With lossy codecs, a higher bitrate usually leaves the encoder more room to represent complex sound, but it also creates a larger file.
There is no universal perfect value. Speech, acoustic music and dense electronic music do not stress an encoder in the same way.