What does 'enable data service' mean on a cell phone?

Without data service, you are limited to phone calls and text messaging.

Data service is needed for web browsing, maps, email, etc.

Maybe you are paying for it but it is not activated?

The phone doesn’t inherently know that you have a data plan. Others with that phone may not. The option you are bing presented with would be useful if you were charged per usage or were in another country and trying to avoid overage charges. If you have an unlimited data plan than just enable it.

If you want to use web services you have to enable the data plan which means activate it.

Nevermind, I think I inadvertently turned data services off, which was why it was asking me to enable. Thanks!

You are authorizing charges that will appear on your bill. Reject it and call your service provider to block further requests.