I don't understand what to do after Library Check

I recently updated calibre to version 6.4, I did a library check, and it says there are no errors. Then it asks me to check if there are discrepancies between library and database, and I get quite a few errors.

https://i.imgur.com/IsqFZCb.png

Now, I did reads the Help section, but it's still really difficult to reallty understand what are my options here. Some errors relate, for instance, to Invalid Title, but the only option given to me here is to delete the book from my library. Why is that the case? How can I "sync" library and database?

I am just a bit confused and not fully understanding the implications of these errors.