After Reading lot of tutorials i get the precise learning for extension attribute.
Let me define it first.
Extension attribute is all about getting the additional information like from checkout page which can not be fetched by magento attribute.
Every interface that extendssdf
\Magento\Framework\Api\ExtensibleDataInterfacecan be extended by extension attributes.
They mostly used to extend functionalities and often use more complex data types than old magento custom attributes.