MP3 is lossy, so editing or re-saving it repeatedly degrades the sound. FLAC keeps full quality at about half the size of WAV. MP3 is at home in music and podcasts that must play anywhere; FLAC is the usual choice for archiving music at full quality. FLAC cannot bring back detail that MP3 compression already removed; it simply stores the result without further loss, in a larger file.
The defaults suit most files. Under Options you can trim the start and end.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine.