From CryptoCurrency Wiki

  • The nonce is simply a random number that is added to the block header for no other reason than to give us something to increment in an attempt to produce a valid hash. If your first attempt at hashing the header produces an invalid hash, you just add one to the nonce and rehash the header then check to see if that hash is valid.
  • The nonce is the value that is altered by the miners to try different permutations to achieve the difficulty level required – the nonce has been appended by the extra nonce function which sits in the coinbase transaction – or the first transaction of the Merkle root signifying who to pay the block reward to – an extra counter to add permutations to as the nonce number can be used well within a second by modern mining equipment.
  • As the Bitcoin mining network has advanced from Mega hashes to Giga Hashes to Peta hashes – the nonce, or the number that the miner varies can be run through in less than a second, which has 4 billion attempts has become easily exceeded resulting in miners having to use an extra nonce factor.