The zoom level controls how large the map is rendered. A level of 1 would render the map at full size, where 0.5 would render at half size. Anything above 1 would cause upscale and therefore a lower quality render, so it's best to stick to a maximum zoom of 1 and create fairly large sized sprites, then set an initial zoom of more like 0.5 to give a more reasonable initial view.
In most cases, you'd want to just set a minimum and maximum zoom level and just enable pinch zoom with the enablePinchZoom() function. This allows the player to pinch in and out with two fingers but requires that they have a touch screen device.