At this moment in time, the blockchain is kept up and running by anonymous peers on the network, known as nodes. As a way of keeping the network safe at all times, the Bitcoin protocol requires each block to prove that significant work was invested into mining it, in order to make sure that untrustworthy peers who wish to modify past blocks will have to work much harder, when compared to normal peers who simply want to add new blocks to the chain.
The process of chaining blocks together has the scope of making it impossible to modify transactions in a certain block, without modifying all the afferent chained ones. Thanks to this, the cost of modifying a particular block increases with each additional block added, thus enhancing the total effect of the proof of work concept.
It’s important to keep in mind that new blocks are only added to the blockchain is their hash has a level of difficulty as the one expected by the current consensus protocol. There are certain mathematical algorithms that tend to change the difficulty of mining a block, depending on how long it takes to mine a certain number of blocks. To put things better into perspective, if it takes less than 2 weeks to mine a total of 2,016 blocks, the expected difficulty will be increased proportionally, whereas if it takes more than two weeks to mine the blocks, then the expected difficulty will be decreased.
Based on everything that has been outlined so far, bitcoin proof of work represents an interesting concept meant to make bitcoin harder to hack by those wishing to change the history and future of the digital currency, while also making users rest assured that the bitcoin they send or receive is actually real.