A mining pool is a service where many users "pool" their resources together to "mine" for cryptocurrencies.
Most cryptocurrencies are so difficult to solve ("mining", which is essentially solving the equations necessary to secure the blockchain and include its transactions) that for smaller participants, it's more realistic to share their resources together, and then for the mining pool to split the rewards out based on the effort each participant executed.
Many mining pools will also provide cryptocurrency wallets for their users, to allow you to accumulate your earnings out over many different blocks and send the rewards in a single transaction, which is often cheaper.
You can add a Mining Pool account to your portfolio by clicking on the Pool button when configuring it.
For more information, check out these resources: