I think what it is saying is that if you want to only draw/paint/color within an area already defined on another layer (such as airbrushing some dots over a desert area where you have that desert on its own layer - the term Base Layer usually refers to the layer where the area is actually defined) but want that paint/color on its own layer you do the following steps:
1- go to the layer where the area is defined (select that layer from the layer menu)
2- Using the magic wand, or if the area is defined by a solid color, the color selector, select the area that you want to paint in (there should be some "marching ants" around the area you want)
3- select the layer that you want to paint on (you should still see the "marching ants")
4- With the "marching ants" still showing, paint away....you should not be able to place anything outside those ants.
5- once you have painted to your hearts content, make sure to deselect the area.
I hope that is clear enough...although it might not be..... :/ ask away if not.