Peak vs LUFS Meter for Mastering

Peak Loudness looks at the highest moment in the whole audio track. If you tried to raise your track up to 0dBFS using the total peak loudness as a reference point, the loudest single moment in the track will be optimized, but the quieter segments wouldn't even come close. This is the same as choosing the normalizing option in your DAW and is not appropriate for mastering. 

LUFS is a generalized standard to keep the perceived loudness of audio tracks the same. This is called loudness normalization and it stops the consumer from constantly controlling the volume when listening to music. 

The music industry is following suit, and now many leading streaming platforms such as Spotify, YouTube, and Apple Music are normalizing music so that the playback is at a consistent volume. If you listen to a dynamically mastered jazz track, followed by a heavily compressed dubstep track on say Spotify, they will maintain a relatively consistent perceived loudness.