Proof of work describes the process that allows a blockchain to remain robust by making the process of "mining", or recording transactions, difficult.

Mining involves a computer-intensive effort that requires the would-be validators to use their machines to generate a winning fixed-length code before anyone else does.

By forcing validators to expend some form of energy to discover new blocks, the idea is it dissuades potential bad actors from participating in the network and attempting to corrupt the blockchain with invalid transactions.

Proof-of-Work is a type of Concensus Mechanism

What makes a block invalid?

A miner can create an invalid block in many ways (such as putting an invalid timestamp on it) or including an invalid transaction in the block.

What happens if a block contains an invalid transaction?

The invalid block would be rejected immediately by any nodes that the miner tries to upload it to. Those nodes would both refuse to add the block to their blockchain and refuse to relay the invalid block to other nodes.

What makes a transaction invalid?

• Spending more money than is available within a wallet (address) or attempting to spend coins twice.
• Transactions with low or no transaction fees