In foreign exchange, every transaction has a counterparty that is responsible for fulfilling the other end of an agreement.
Aggregate risk is the risk of an institution having a large number of agreements with just one counterparty. This leaves the institution in question exposed should that counterparty be unable to uphold their side of those agreements. Aggregate risk is easy for institutions to remedy.
It merely calls for diversifying counterparties to reduce the risk of any one of them proving to be unreliable.