Good morning.
Hz refers to frequencies output by your speakers. Higher frequencies are things like women's voices and birds chirping. Lower frequencies are rumbles, men's voices and subwoofers. (You probably knew that, but I wanted to cover everything.) Hz by your auto setup is like doing what's called determining crossover. Your speakers generally don't handle all frequencies well. Most handle higher frequencies well while letting your subwoofer play the lower frequencies. Your receiver was likely determining the cut-off point for those lower frequencies (usually around 80Hz). Frequencies below that cut-off point (exists in most movies and a lot of music) go to your subwoofer, as the subwoofer is designed to play low frequencies. Frequencies above that cut-off point are played by the other speakers in your setup. Volume/loudness is usually represented with decibels=dB. Your receiver probably also did loudness matching to make sure your front speakers aren't too soft, and your back speakers aren't too loud for optimal listening.
Welcome to the forum.