What does overage data mean in regards to an iPhone?

I just got an IPhone and I got a message saying I have used 75% of my 300mb overage allowance for bill cycle ending on christmas. I don't know what overage is or how I ma using it, like it is because I use snap chat? Please help!

Added (1). I have at& t if that matters

Hi its self explanatory really basically what this text is is its pre warning you that you have used the majority of your data allowance for the month I. E you have 300mb per month to use but you've used around 225 mb so there just pre warning you so you don't go over your allowance for the month

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