I am trying to import sql database and receive this error message. What does it means?

MySQL menyatakan: Dokumentasi

1452 - Cannot add or update a child row: a foreign key constraint fails (wwwsoulf_revsoul2.timeline, CONSTRAINT timeline_ibfk_1 FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE CASCADE ON UPDATE CASCADE)

I can't understand this syntax:
CONSTRAINT timeline_ibfk_1 FOREIGN KEY (user_id)

what is timeline_ibfk_1 ?

No, the foreign key is working .

Have you read the documentation? If you do and still don't understand I'll tell you how to fix it, but you clearly haven't put in any effort.