What is meant by "a codon codes for an amino acid"?
It does not means "provides instructions for manufacturing an amino acid".  How could three little letters encode so much?

A codon is a three nucleotide sequence that matches the anti-codon of a t-RNA molecule capable of carrying one kind of amino acid.  The GGG codon, for instance, binds the t-RNA that binds to glyceine.  So the codon GGG in an mRNA ends up amounting to "put a glyceine here in the amino acid chain"