There are a number of ways in which an import file may trigger an Error or a Warning particularly if the JSON file was created by a third-party tool.
When Ghost detects an error, the import will be cancelled, no information is imported and there should be a suitable error message to help you debug the problem. This also means there may be multiple errors in your import file but you will only see a single error during the import process.
An Error or warning should contain a relevant message on why the import was not successful along with a the relevant JSON entry that caused the issue where applicable.
An example of an error would be a user that contains a null email field:
The import may generate multiple warnings. These are issues that may want to know about but are not significant enough to prevent the data being imported. Examples of this include a duplicate user (either duplicated in your JSON file or matching an existing user) or a post linked to an unknown user.