As we are aware that blockchain is decentralized network which is focused mainly on transparency, Blockchain is list of growing records titled as blocks.
Please refer this article, if you are beginner in blockchain domain.
Role of Miners in Blockchain :
Whenever transaction is made on bitcoin network transaction needs to get verified to add that transaction into Blockchain. Miners are responsible for verifying transaction. Miners may or may not be node in Blockchain.
Miners solve typical mathematical logic to verify transaction, many miners will be competing among themselves to solve logic, reward will be given to miner who first solves logic, this is also called as Proof-of-Work which is consensus mechanism.
When transaction is made by node, initially it is noted as unverified transaction, all unverified transactions and pending transactions will be stored in memory pool, from memory pool unverified transaction will be picked by miners and they compete among themselves to solve logic, one who solves logic will be rewarded and same miner will add that verified transaction to Blockchain.
Which type of block is referred to as Candidate Block :
Consider there are 10 blocks in Blockchain, at an instant new transaction has arrived which needs to be validated by miners before appending it to existing block chain, this new transaction will be stored in memory pool (this new transaction cannot be added to blockchain until verified by miner) along with remaining unverified transactions, 11th block is referred as candidate block which is created by miner, in this block unverified transaction from memory pool is picked and miners will compete among themselves to solve it, after solving it, this becomes verified transaction and will be added to Blockchain. Now 12th block is called as candidate block and same process goes on.
Though, miner generates candidate block, there is no guarantee that same miner has ability to validate block, any miner can validate block.
So, candidate block is referred as one which miners are working currently to add it to block chain.
Candidate block is temporary block it will change overtime, block could not be treated as candidate block permanently.
A genesis block will be treated as genesis block permanently as it is first block of block chain.
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