A "Layer" is a set of information or data. For instance, one layer could be 'counties' in New York State. Another layer could be 'interstate highways' in New York. These layers are laid on top of each other and are visible as a single map image. Add more layers about cities, rivers or other information and you get a more detailed map displaying more information.
The Map Layers List is used to make more or less detail visible. It lists the data layers in the current view of the Map. Certain layers will only appear when you zoom in to a certain scale. Beside each layer name is a check box. Checking the box next to a layer will make that layer visible and unchecking it will make it invisible, but you must first click the "Refresh" button. You must click the "Refresh" button every time you change the visibility of a layer.