Bitcoin mining is founded on the Proof-of-Work algorithm. This algorithm demands that miners resolve a complex mathematical problem to submit their verified transactions to a block chain to get Bitcoin rewards. So the Proof-of-Work is essentially an economic measure to prevent DDoS attacks and other such service abuses like spam on networks. Its goal is to prevent cyber attacks like the DDoS attacks or Distributed Denial of Service attacks which can drain a server of its resources by flooding it with fake requests.
This PoW concept had been there even before the advent of the Bitcoin. Satoshi Nakamoto had applied this to the digital currency and thereby changed the manner in which traditional transactions used to be carried out. The PoW is a requirement for defining a costly computer calculation which is known as “mining”. This must be done to ensure trustless transactions of “block” on distributed ledgers or block chains. In a PoW algorithm, the miners will have to make many guesses to solve the puzzle to get the right combination. This demands a lot of time. To keep nodes operational to guess this combination, you will also need uninterrupted power supplies and internet connections. All this entails high operational costs. Machines being run for long periods will emit heat and so, cooling machines are needed to prevent fire outbreaks.
The very first miner which can guess this combination rightly is liable to submit the block of verified transactions to a block chain. He will be rewarded with Bitcoins. This key or combination is what is called the Proof-of-Work that needs to be given to block chains.