What does this yellow ! mean?

QQ拼音截图20210517163721

See attached.

It is a warning about the title syntax.
Put your mouse pointer on the orange triangle for a few seconds, and JabRef will show what is it about.
For example: capital letters that are not enclosed in curly brackets.

These warnings can appear for other fields, and help you keep your library tidied up.
A Quality -> Check integrity gives you a list of all these little issues for your library.

Thank you. I see.
By the way, why do my other questions which are proposed before have not been answered so far?

I guess that nobody took the time to answer you yet. :sweat_smile:
We (users and developers of JabRef) are all volunteers, and so we do this on our free time.
Note that looking at the doc may help you (https://docs.jabref.org/).
Cheers.