It's the potential difference between the inside and outside of the membrane of the cardiac myocyte known as our sarcolema. So the sarcolema is another word for the membrane of a cardiac myocytes. So basically, this is given in millivolts. And as we go higher along the y axis, our voltage increases, it becomes more positive. Now, the x axis is our time and that's given in milliseconds. And as we go along the x axis in this direction, our time progresses, our time increases.