In a mining pool, several entities join together to "mine" a cryptocurrency.
A Bitcoin mining pool bundles energy or computing power, for example, to increase the chance of jointly generating a block and receiving the block reward for it. This is then divided fairly among the mining pool members.
However, a mining pool account by itself is not enough to mine Bitcoin successfully. In addition to the appropriate hardware (e.g., the so-called ASIC miner), a mining client is usually also required, which, among other things, saves the cryptocurrency in a wallet. The previously created mining pool account is created in this wallet.
Among the best-known open-source mining clients are Bitcoin-Qt, Electrum and Armory.