incoming payments

Hi,

when there is incoming payment, system does a DR posting on the Cash Account. What does this mean? What is happening in this process? Kindly explain in layman's term.

Is the Cash account : quote internet "u2022A type of account with a broker-dealer in which the customer agrees to pay the full amount due for the purchase of securities within a short period of time, usually five business days." ?

Must there be a corresponding CR account posting?

regards

John