What exactly is bandwith

1

K I have never realy known the exact definition of bandwith. Like my internet connection says I have a max download of 400 MB and if I go above that its 1 ruppee (indian currency) per MB. So I didnt donload mch at all. But i did do a lot of surfing. and then I see my bill and its this huge number. So I mean what exactly do they mean when they say my max bandwith is 400MB? plz reply fast