A raster paint layer is a canvas into which you draw strokes. The strokes are merged together as they are drawn to produce the resulting raster image. Each layer has a front and an alpha. You can select the one on which you want to draw by selecting its proxy image in the Layer Editor.
For example, the following graphic shows that the layer named “University” is selected and its front image selected for painting (it has a white frame around its proxy image).
