FLAC (which stands for Free Lossless Audio Codec) is a way to code audio files. It makes those audio files smaller, without affecting the sound quality. It is unlike MP3, because when encoding audio using an MP3 encoder, information is lost and cannot be recovered. When a FLAC file is decoded, it will be identical to the original raw audio. FLAC is the most common lossless audio codec.
Lossy encoders, such as MP3 encoders will generate smaller files than lossless encoders such as FLAC, although with a lossy codec the decoder's output will not be exactly identical to the encoder's input, although with a decent encoder at sufficient bitrate, it will be extremely close.