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 OPUS?
Opus is the best-sounding lossy codec at low bitrates by a comfortable margin, which is why it carries the audio in nearly every voice and video call you make. It is royalty-free, it handles speech and music equally well, and it is usually wrapped in an Ogg container.