Can someone explain exactly what roaming means. Does it mean I am out of my hometown or out of the service area? Also I have the $39.99/month plan with at&t. Does anyone know if there are roaming or long distance charges? I checked the website and it was not very clear about this.

Roaming means you are not using your carrier's network.

For example, I use Sprint, but there may not be a Sprint tower where I am traveling. However, Sprint (like other carriers) has roaming agreements in place with with other carriers to allow me to connect to the other carrier networks - like Verizon.

On the AT&T plan, you will need to make sure you have nationwide roaming for the voice plan (I assume you are speaking of voice, not data). You should be ok, I think.