"Mining" is a term associated with computer hardware that runs algorithms in order to find 'blocks', which are sequences of data. The 'mining' process, in short, revolves around finding patterns of data using a hash algorithm. Miners will make their hardware, which is usually GPUs or ASIC miners run a hash algorithm that searches for 'blocks' of data, that when found, the algorithm is applied to the block and a sequential pattern of data is the outcome. This rewards the miner with cryptocurrency such as Bitcoin; the whole mining process is purely mathematical. Bitcoin is worth a lot of real life currency.
Back in the old days, mining was commonly done on the CPU. The CPU was very inefficient when finding blocks, so GPU utilization was implemented for mining - it proved to be much more efficient compared to CPU mining. In short, the GPU is used by a mining software that runs a hash algorithm to find blocks. Many miners mine in pools because whenever a block is mined in a pool, the reward is shared to everybody in prioritization of computing power.