FLAC is a lossless audio format. A lossless audio format contains all the audio data produced or recorded without any compression or alteration to it. FLAC files are usually much larger in comparison to a typical MP3 audio file.
FLAC version of an audio are typically used by industry professionals who deal with high grade audio processing such as a music production studio.
A lossless audio is to music like a RAW image is to a photograph.