What is end-to-end encryption, over which WhatsApp threatened to exit India?

End-to-end encryption allows a message to be read only by sender and recipient. When users send a message through WhatsApp, it's secured with cryptographic lock and only the recipient has its keys. Even WhatsApp can't read users' messages due to encryption. WhatsApp is challenging an Indian law that would require it to trace and identify first originator of a message.