A closed currency is a currency that you can (normally) only obtain in the country where the currency is used. You cannot take out the currency from the country as well, meaning you will have to use all your currency or exchange it for your own currency before departure.
Here is a full list of countries with a closed currency:
Although some countries don’t allow any currency to be brought in, there is some flexibility.
For example, you can bring in or take out 1000Dh, for any extra you will, however, have to change the money in Morocco.
If you are travelling to a country with a closed currency please check before you travel as some countries such as Bahamas will accept US dollars.