Data Roaming - Define Significant Charges

I've always kept data roaming turned off because if turned on I get a message that I could incur "significant charges."

Now, due to my kids moving out of state, we drive between DFW and IND three or four times a year. Without data roaming, I get no phone service between major metropolitan areas.

I would like someone to define "significant charges" so I'll know what to expect if I turn data roaming on during these trips.

Also, what woudl cause the charges? Phone calls, playing games that require internet access, using the google maps and search engine, any or all the above.

I'm just kinda clueless about the whole data roaming thing!

Thanks!