system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening differently in a person's body that sings very well versus a person who is a horrible singer?
gpt
There are different aspects of singing well. The most important are timbre and audiation. If someone is singing in tune, but it still sounds bad, that's a timbre problem. They probably need to relax and breathe more consistently.

At another level, the way your voice sounds is also determined by the arrangement of your larynx and the bones in your head.

The other perhaps more common problem is a failure to audiate well. Audiation is to music, what thinking is to language. To sing in tune you have to exert the effort to THINK the sounds you want. It's not difficult to practice but it can be mentally exhausting.

Any musician who improvises well is audiating, as is anyone who composes music straight to a score without playing it.