A Sender id is a name or ID with which your message that is triggered through Kaleyra's platform will be seen when the SMS is received on the handset. 

This is usually composed of 6 characters. 

With advancements in the SMS industry, it is now mandatory to register your relevant senderIDs with the DLT platform before you start using them, as per the Telecom Regulatory Authority of India (TRAI).