What is FLAC?
FLAC is lossless: it compresses audio to roughly half its size and gives back the original samples exactly, bit for bit. It is what people archive CD collections in and what high-resolution download stores sell. Sound quality is identical to the source; file size is not.
What is MP3?
MP3 is the format that made digital music portable, and thirty years on it is still the safest bet that a file will simply play - on any device, in any car, in any decade-old piece of hardware. It is lossy, and its patents expired in 2017.