Cubeb audio sink

What does it do and what does it fix?

An alternative audio backend that features lower latency.

You can read about it in more detail on Dolphin’s blog post from last year: Dolphin Emulator - Dolphin Progress Report: May 2017

Could this be apossible fix for x/y as they said the error was because of unimplemented audio functions

No. This is only responsible for emitting sounds Citra has processed, but doesn’t affect sound processing in any way.

What does latency mean? How is that better

In general latency means the time till something happens or is done.
Lower latency = faster response time = less lag