This is the ability for one to access their mobile network service whilst out of their country. If you are an Econet subscriber and actived roaming on your Econet line, when you travel out of Zimbabwe you will still be able to use your Econet sim card to access services from another network (whichever network Econet has an agreement with that country). This will then give the ability for people in Zimbabwe to call you on your Econet Zimbabwe number and you will answer it whilst out of the country, say in USA or even UK!
This is normally done before leaving the country by either dialing a USSD code or sending a text message to a certain number (basically you have to register before leaving the country) and it is FREE of charge to register. However, the network will bill you for receiving calls and text messages.
These charges are normally high and it is usually best for you to just register a number in the country that you are going to or if it is really paramount that people still be able to contact you on your Econet number whilst out the country, then be very carefully of which calls you answer.