Next to the validation error icon you will see three dots, Click on it to download the error report. Errors can be off: invalid phone number, additional field values are not matching, not a valid email id, the number is a duplicate.
1. Only when you want to import a contact along with a tag or add a tag to an existing contact, you can use simple import. 2. When you want to add custom additional field values, then you can use advance import contact