in simple terms - the pattern is printed onto paper or any other foundation material and you place the fabric over the area you are working on and sew on the line to do the piecing. it allows for detailed precision work.
its great for working with points, odd shapes, and you can also use it to work with fabric that normally wouldn't sew together well.
some times the foundation is taken out after the piecing is done and in other cases the foundation is left in.