A "mask" is something that decides which parts of a layer are shown and which are hidden. So if you have a colour layer in front of a B/W, you can use a mask to define which parts of the colour layer are shown, and which aren't.
Instructions at: http://www.essential-photoshop-elements.com/layer-mask.html
- Create new adjustment layer
- Position adjustment layer between layers
- Select top layer and group with previous (which is the adjustment layer)
- Paint with black on adjustment layer to hide areas of the top layer