Custom fields are useful for adding data to your products. This can be done manually for individual products or in bulk.
To create custom fields manually, go to 'Products' and click 'Edit' for a product. Scroll down and add a custom field. Then click 'Save'.
To add custom fields in bulk, create a Google Sheet/Excel with a column for EAN (with your EANs) and other columns for the custom fields you want to add.
When importing, only map the 'EAN' field and the rest will be automatically imported as custom fields.