A nonce is an abbreviation for "number only used once". It is a number added to the hash of the block in a blockchain that, when hashed, meets the difficulty level restrictions. The nonce refers to the first number that a blockchain miner must discover before resolving a block in the blockchain. It is difficult to find and is considered a way to eliminate miners with low computing power.
Going around the solutions to guess the nonce is considered Proof of Work, and the miner who is able to find the value is awarded the block and rewarded with cryptocurrency. It is very unlikely that a miner will manage to guess the nonce on the first try, which means that the miner can potentially test numerous nonce options before getting it right.
The greater the difficulty, the more likely it is that it will take time to figure out a solution. Block difficulty is the same across the network, which means that all miners have the same chance of finding the right hash.
Blockchains set a target number of blocks they want to process over a specific period of time and periodically adjust the difficulty to ensure that this target is met. If the number of blocks processed does not reach this target, the difficulty will be reduced, with the reduction in difficulty set to the amount of time beyond the processing time limit.
