IN PLAIN ENGLISH:
In order to make the whole cryptocurrency marketplace work, a large number of computers with a lot of computing power is needed to host the blockchain and validate transactions. To incentivize people to dedicate their computing power to this, a reward is offered.
The technical details:
In the case of Bitcoin, a new block is created every 10 minutes that contains all the transactions that have taken place during that time. Miners on the network compete to be the first to solve a complex math problem. Once solved, they announce the solution to the network where other miners verify the solution is correct, and that all transactions on the block are valid. If everything checks out, the block is added to the blockchain, the ledger is updated with all the transactions it contains, and the miner that solved the math problem is given the reward, in the form of Bitcoin in this example. Many other cryptocurrencies operate the same way, although some are not mineable.
When it first started, many people began mining bitcoin from their homes. However, over time the mining process has become more challenging and competitive. There are now huge mining farms, with thousands of processing units running to try to be the first to solve the math problems and receive the reward. It is now very expensive to try to compete in mining, which makes it challenging for the average person to get involved in.
