[plt-scheme] Re: Explanation of this piece of code?

The quality of that code isn't great, but you're asking about very basic 
features of Scheme, like "let*".

If you're already comfortable with programming in a language other than 
Scheme, and are in a hurry, I suggest reading through one of these two 
documents:

http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme-Z-H-1.html

http://schemers.org/Documents/Standards/R5RS/
(You can skip the parts about continuations and formal semantics in this 
document.)

Either document will not only give you a quick intro to Scheme, but 
should answer almost all your questions.  Give one of them an hour.

The one question those documents might not answer concerns "filter".  
For an illustrative example of "filter", which will make sense after you 
understand "map", see:
http://srfi.schemers.org/srfi-1/srfi-1.html#FilteringPartitioning

Someone could walk through and explain the code example you posted, but 
I think you'd understand more of that code in the same amount of time by 
going through an introductory document.

-- 
http://www.neilvandyke.org/