The HTML5 canvas has X-Axis and Y-Axis and is two dimensional. It has starting point in the upper-left corner which acts as origin for any element, action that will be part of canvas. See picture for more details:
You can see that coordinates positioned at the upper left corner and X and Y axis points are set to 0 pixels. Lower right corner has been set to 16 pixels. Any item that will be part of this canvas is going to be positioned with accordance to this coordinates of the HTML5 canvas.