"The Bitcoin nonce forms part of the block header, which is used by miners to provide entropy as part of the Proof of Work process, to try and find a hash meeting the difficulty requirement."
Quote taken from the following article.
https://blog.bitmex.com/the-mystery-of-the-bitcoin-nonce-pattern/
It means randomness. In computing, if something a source of entropy means that means it is a source of randomness.
In order to prepare a valid block header, transactions from the mempool should be selected, merkle root should be computed, block version, previous block hash and difficulty value should be filled. Finally, time should be set and with nonce=0 hash calculations will start.
As far as I understand, what the author meant was: by incrementing nonce you continue to search for a valid hash, i.e. provide entropy. nonce is the fastest mechanism to try a new hash, until it maxes out.
Obviously, mining hardwares today uses mechanisms like extraNonce but it is a whole another issue.
