What Is “Extranonce” and Why Is It Necessary in Bitcoin Mining?

Extranonce is a field within the Coinbase transaction of a block, which is part of the Merkle Root. It is used as an additional variable to iterate through when the primary 32-bit nonce in the block header is exhausted.

Since the hash rate is so high, the extranonce allows miners to change the Merkle Root, thus opening up a massive new range of hash possibilities.