Layer modes are a mystery to any beginner to digital imaging and there are more complex exercises in the Improvers level course, but there are quick and simple exercises that can demonstrate how layer 'modes' can be used to fix common image problems; this one deals with over exposed or slightly 'washed out images.
To explain briefly, using a layer 'mode' defines how the pixels on the layer to which you apply the viewing 'mode' affect or 'act upon' the pixels in the layer below - crudely a bit like using coloured sheets of film over a print.
We'll use the 'Multiply' mode to enhance a landscape image taken in bright sunlight.
This picture of a summer pasture in the Carpathian mountains of Romania is a good example of an image that is somewhat 'washed out' due to being taken into the sun on a bright day.
Download this image as you have with all the task images so far, using the link above. When the picture is open, make sure you have the Layers palette visible as well.
Step 2 - Duplicate the image layer 
Duplicate the background layer, using the third icon from the left at the bottom of the Layers palette (arrowed left).
Tip: if you don't have the Layers pallette visible, the shortcut key combination CRTL +L to open it.
To achieve the effect, make sure you click on the newly duplicated layer to select it.
Now change the the 'Blending Mode' by using the drop down list (arrowed) to select 'Multiply'.
What is happening is that you now have an effect similar to two identical slide exposures laid on top of each other.
Tip: If you want to adjust the strength of the blend, adjust the 'Opacity' setting slider just below where you used the drop down list for the layer mode. For this image a setting of around 75% is probably about right.