ELI5: Sound quality

So I understand video quality and resolution and everything like that. I understand that 1280x720 and above is technically classed as HD with 1920x1080 sometimes being called "full HD".

But what about sound? I'm curious. What makes it sound better? How can I identify superior sound quality when comparing two things? How does the hertz thing come into play? Like 720p is HD for resolution, what's defined as HD for audio?

In general could someone ELI5? Would be great, thanks.