If the fields in the imported file do not equal the fields that are expected, the import file will fail.
Some fields can be set to be required. If fields are required, the required field can not be empty, if so the import will fail.
Pressing variable fields to the left it is possible to see the expected fields in the import file.
An import file can be skipped in case that the import last import file has no altered fields.
An import file can be partially imported. That means that not all lines in the import file are imported and the skipped lines can be found under import details. It is found by pressing the Products and in the bottom press Product import history.
Right click on the import file and choose import details