Think about your stereo and the tone controls. One is for bass, the other for treble. You know what those do. Picture a third control between the two that adds or subtracts from the midrange tones of the music rather than highs or lows, depending on whether you turned it up or down. That would be the "mid" part of the thing.
The "sweep" means that rather than the manufacturer deciding for you which frequencies he will set the control at, which you can boost or cut, there is another control, a sweep, which you can turn to either boost or cut a higher or lower frequencies. It gives you more precise control over the specific tones you are editing. Hope this helps.
Plug-in's? I don't know much about digital recording but I suspect the equalizers are pretty much a built-in feature of most systems--someone please correct me if I'm wrong.