Yasu recently summarised what traits are for very well in some other feedback:
when building your character, think of what a complete stranger would say when stumbling upon them.
There is no advice like this on the character editing page though and there should be something just like it.
I can approach trait with a mentality that I'm describing what my character really is, but this site works best when I describe what others would recognise, because that's best for searching.
Usually those are the same thing, but you're an ageless deity or an immortal elder vampire, and you look and act like you're 30, it is easy to be unsure what to pick - until you recognise traits are for people finding you by, and the important thing is usually you look 30 in those cases.