Paypal can hold the payment for a number of reasons; however, the most common one is that the customers funds need to be verified before they can be transferred to you. Paypal should give you some type of notice within your admin on why the hold is there, but should also show you when the hold is released. It would be best to avoid fulfilling an order until the hold is removed, and the funds are paid out to you. Doing so will ensure that you're not losing your product should the payment not clear.
You will need to speak with Paypal as only they can provide further support on removing the hold.