Mining is how proof-of-work blockchains like Bitcoin validate transactions and add new blocks: miners compete to solve a computationally expensive puzzle, and the winner adds the next block and receives a reward. This process is also what issues new coins into circulation on networks like Bitcoin.
Mining requires real hardware and electricity, and profitability depends on equipment costs, electricity prices, and network-wide competition. Investment pitches that offer "guaranteed" mining returns with no verifiable hardware or operation behind them are a common scam pattern.