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.
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.
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/).