A characteristic or property of a product that describes some aspect of the product. Magento users can create custom attributes to add to the default attribute set or a custom attribute set. Create these attributes through the Magento Admin or programmatically. Examples: color, size, weight, price, age, gender, etc.
Custom attributes are a type of Entity-Attribute-Value (EAV) attribute.
For integrations like Google Shopping ads Channel and Amazon Sales Channel, you map Magento attributes to attributes in the third-party to properly display and sell products, display ads, etc.
Learn more: EAV and extension extension_attributes