Your rudeness is entirely unnecessary and unworthy of this site. Not everyone knows the details of audio formats. That's OK.

OP – the reason they state this is that FLAC is lossless so offers limited compression but preserves input audio exactly. AAC is a lossy compression format and will discard input audio data that it thinks you don't need – converting it to other formats may produce artifacts. OGG is a container that supports both lossless and lossy internal formats, but I believe it's typically used with a lossy internal format (called Vorbis).