Internet usage charges, 5GB- what does that mean?

[sarahlily89]

I have a plan with At&t using the Mercury connect internet card. It says that I have 5GB included in the plan to use over a month. I do not understand what this means. Is that for downloading, or using the internet in general? I am worried because I purchased and downloaded a program from the internet that was about 500MB ( what ever that means), and I am worried that I overused whatever internet usage I had. I have to use this internet system because I am out in the country and its the only one that gives fast service, so I would like to understand it better. Thanks.