Bitcoin mining is the process of updating the ledger of Bitcoin transactions known as the blockchain. Mining is done by running extremely powerful computers (known as ASICs) that race against other miners in an attempt to guess a specific number.
The first miner to guess the number gets to update the ledger of transactions and also receives a reward of newly minted Bitcoins (currently the reward is 12.5 Bitcoins).
Today, in order to be profitable with Bitcoin mining you need to invest heavily in equipment, cooling and storage. It’s not possible to mine profitably with a PC or a GPU at home. You can calculate your profitability using a Bitcoin mining calculator.