If you mean what the actual definition means, it's about using (normally external) factors that would normally indicate someone's health status; someone with cancer and going through chemotherapy would have external factors that indicate their poor health, such as the loss of hair, pale or chapped skin, losing weight quickly, looking nauseous, etc.
When it relates to beauty/looksmaxxing/etc, sometimes you will have a bad 'health indicator' that is not actually being caused by an underlying health issue, but it
looks like it is or is traditionally associated with a health issue, and/or it's about improving your appearance to make it seem like you're a healthy individual whether you are or not.
For obvious reasons, most of the time these symptoms
are caused by an underlying condition, so it makes sense to treat what's the cause.
@neymar's list is a good list because it displays what symptoms are traditionally associated with health. His list is actually good for both ways. As an example, nails can be an indication of good
or poor health.