Roaming means when you are in an area and you can't get a tmobile data service/signal, it will connect to some other service provider so that you can still get data usage.

Its like when you go to another country and your phone connects to a foreign network, thats roaming. But its EXPENSIVE. Unless you want possibly VERY HIGH bills leave it turned off.

 

Possible I'm not entirely sure. I know that the service provider you "roam" onto has to have an agreement with your own service provider. I would think its best to keep it off just to make sure. T Mobile should be able to confirm this better than me tho

 

Data roaming should relate to using internet services whilst abroad specifically - this can be pretty expensive, so only use when necessary or if you are rich

Tmobile charge