Data overage is simply using additional data than what is allotted in your plan.
You will receive data notifications alerting you of your usage.
You can use up to 20GB in data overage.
Your data speeds will be very slow; you can upgrade your plan or wait until your data plan renews.
Data is $0.005 per mb
Ensure to pause/cancel Software updates until you are on WI-FI, as a large amount of data may be running in the background. IOS/Android users can turn background data on or off from the settings. In order to do this, select "Settings", then tap the "Cellular" Icon. Here, you can turn on or off the 3G/4G LTE or Data Roaming functions, and if you swipe to the bottom of the screen, you will see a list of apps to turn on/off