Mining Bitcoin or other cryptocurrency involves solving complex mathematical algorithms and in return the miners are issued a certain number of cryptocurrencies as a reward. The reason it is called mining is because just like mining gold or other natural resources there is a finite amount of Bitcoins that can be mined. The maximum amount of Bitcoins that can be mined is 21 million, other cryptocurrencies have different maximum supplies.
Bitcoin mining requires a large amount of computing power to solve the required algorithms and as more miners join the network the more difficult it becomes to mine. When Bitcoin was first released in 2009 miners used computer processors (CPU) to mine which meant anyone can mine Bitcoin using their computer or even laptop but as more miners join the network the difficulty has increased and miners moved to mining using graphic cards (GPU) as they became more powerful than processors. Now the difficulty has increased so much that special computer chips were designed specifically for Bitcoin mining called ASIC (Application Specific Integrated Circuits) miners.