whats a closed currency and what does it mean?

It means that you can't get it until you enter the country. If you are visiting a country with a closed currency, you change your money on arrival. When you come home, you will need to change it back again before leaving. You won't be able to exchange it in the UK.

Black Guru is correct

One of the good things though about closed currencies is that you can usually get a better exchange rate on the black market. There are lots of places you can find black market deals but sometimes the penalties are high for changing money somewhere other than an official place.

I could go into it but your best bet would be to ask a taxi driver. Make sure you have cash. US dollars are usually the easiest to exchange as they are so widely known. Although the Euro is now quite good with North African countries like Morocco or Tunisia with Europe being only just across the water.

Rather than asking the driver to take you to the bank or change your money hold up say a 50 dollar / Euro note and ask where you can change it. The reply you'll often get is " Right here Sir / Madam". Just make sure you know what the banks would give you so you can haggle your price accordingly. The best I ever got in one country was 6.5 times the official bank rate by changing it on the black market.

Good luck and have a great trip.