JavaScript: Create rounded rectangle HTML5 Canvas

SimpleZoomPan.js

{

		/**
		 * [roundedRect description]
		 * @param  {Canvas Context} ctx       Canvas Context
		 * @param  {int} x         X Coordinate
		 * @param  {int} y         Y Coordinate
		 * @param  {int} width     Width of rectangle
		 * @param  {int} height    Height of rectangle
		 * @param  {radius} radius    Radius of rectangle corners
		 * @param  {string} fill      Any string of rgb or hex
		 * @param  {string} stroke    Color of stroke --if stroke === 0 it will not use default stroke
		 * @param  {int} lineWidth The line width
		 * @return {[type]}           [description]
		 */
		roundedRect: function (ctx, x, y, width, height, radius, fill, stroke, lineWidth) {
			var useStroke = typeof stroke == 'undefined' || stroke !== 0;
			radius = typeof radius == 'undefined' ? 5 : radius;

			// Save previous context
			ctx.save();

			ctx.beginPath();
			ctx.moveTo(x + radius, y);
			ctx.lineTo(x + width - radius, y);
			ctx.quadraticCurveTo(x + width, y, x + width, y + radius);
			ctx.lineTo(x + width, y + height - radius);
			ctx.quadraticCurveTo(x + width, y + height, x + width - radius, y + height);
			ctx.lineTo(x + radius, y + height);
			ctx.quadraticCurveTo(x, y + height, x, y + height - radius);
			ctx.lineTo(x, y + radius);
			ctx.quadraticCurveTo(x, y, x + radius, y);
			ctx.closePath();
			if (useStroke && stroke !== 0) {
				ctx.strokeStyle = typeof stroke == 'undefined' ? '#000' : stroke;
				ctx.stroke();
			}
			if (fill) {
				ctx.fillStyle = fill;
				ctx.fill();
			} 

			// Restore original context
			ctx.restore();

		}
	};

May you please explain how this works?