Proof of Work is the consensus algorithm, a system that Bitcoin uses to confirm transactions and generate blocks on the blockchain. The data is difficult to produce because it is costly: it uses large amounts of computational power therefore incurring large electricity costs for the work done by the computers and their cooling off. It is also time-consuming: it may take computers a lot of time to do the work and create blocks. The difficulty of the data-production work rises or falls according to number of transactions being made thus the demand for creation of blocks. This means that the more transactions there are, the more difficult the work to create blocks gets and therefore, the more computational power is needed. This is what is called “mining” and the computers which do the work and create blocks are called “miners”.