What is FLAC?
Lossless compression for audio: about half the size of a WAV, with every sample bit-for-bit intact. It's what people archive CDs into and what hi-res download stores sell. If a conversion starts from FLAC, it's starting from a perfect copy.
What is AAC?
MP3's designated successor and the audio in most video - YouTube, streaming services, broadcast, virtually every MP4. At a given bitrate it beats MP3 audibly, especially below 128kbps, and every phone and browser decodes it.