The Layers Dialog is the main interface to interact with Layers present in a GIMP image. You can think of layers as a stack of slides or clothes on your body. GIMP gives you several methods of combining layers with help of modes. The mode of a layer will determine how it will interact with other layers present in the image.
Before you can do anything with a layer, the layer must be activated.  You do so by clicking on the layer name. If you double click on the name you will be able to set the layer attributes of that layer.
If you right click on the layer name you will be able to access the layer menu. Within the layer menu you have access to several other layer commands, see Chapter 5.
With the Opacity slider you control the general transparency of the currently active layer. The Keep Trans check button controls whether or not you can paint on the transparent areas of you layer. If it is checked, you can't paint on transparent areas of the layer (i.e. you can only paint on opaque areas).
The GIMP's Layer Dialog is by default in Auto mode so it will always show the image that you are working with. You can, however, switch this off and choose another image to work with from the drop down menu.
if you only have one layer that layer is always active