When a field in a data extension is marked as Nullable, it means that the field is not required to have a value; it can accept null or empty values. During the import process, if a field is marked as Nullable, it indicates that the corresponding column in the file being imported does not necessarily need to contain data for every record. This allows for greater flexibility in the data import process, as not all data fields may be relevant or available for every record being imported into the data extension.