What is the Meaning of this Notation in the Context of Smooth Retractions?

[CornMuffin]

Homework Statement

Here is the context:
suppose $f=(f_1,...,f_{n+1})$ is a smooth retraction of $B^{n+1}$ onto $S^n$But what does the following statement mean?
$\int _{S^n}f_1df_2\wedge df_3 \wedge ... \wedge df_{n+1}$

Homework Equations

The Attempt at a Solution

 

[xaos]

this looks like it is being used in context of stokes theorem. any more information i think would be doing the problem. i think you can use the most obvious retract for this.