[plt-scheme] what is fold-left?

Morning,

I'm looking at some code at
<http://funcall.blogspot.com/2007/08/naive-bayesian-classifier.html>. This
code by default will not run in DrScheme. The code is using (fold-left
...). Looking at the function it looks like a (map ...), but I don't
(yet) know why I can't just use a 'map' instead of the 'fold-left'.

Any ideas? What does this function do that 'map' doesn't and why
should it be used?

Mike