it means that you have funds inbound (a direct payroll deposit or other electronic transer, i.e. paypal), which have had their source comfirmed, but have not yet posted to your account.
In banking is standard that the department that handles transfer operations is not able to directly post transactions to customer accounts...it's a firewall of sorts. Instead, they post a "memo" to an account indicating that funds have been received and comfrimed. If the department (or electronic system) that is able to credit and debit customer accounts is not open or online, the transaction will not be processed, instead, the memo is noted so that the customer can have access to the funds and the receiving bank can more quickly record the transactions and charge it's transaction fee to the sending banks. It's all about speeding up the flow of funds.