ICO pools bring together investors who plan to invest into ICO's, but do not have enough funds to get approved for a pre-sale on their own. Since most of the promising ICOs sell out already in the pre-sale, these smaller investors have to join forces in order to be able to participate.
ICO Pools can be either Trustless or Non-Trustless :
Trustless pools utilize smart contracts to limit direct control over the collected funds by any party. Investors can check the smart contract address prior to depositing funds, verifying that it's setup properly as it shows the recipient ICO address too ... since it's open source. The distribution of the purchased tokens is then handled automatically via the smart contracts too.
Non-Trustless Pools do not rely on smart contracts, there are pool managers who do that. Investors send funds to the managers address and rely on their honesty and integrity, to provide the service they agreed to. There is usually a fee charged by the managers, to compensate for their efforts.