Intergenic suppression is the suppression of a mutation that has rendered a gene nonfunctional because the coding has changed to call for a wrong amino acid in the final polypeptide product, or even to call for the wrong stopping point.
The mutation is suppressed when another mutation (a suppressor mutation) occurs which creates a mutant tRNA molecule that reads the mutated code as the normal code, and thus puts the correct amino acid into the final polypeptide product, thereby making the gene functional again. The original mutation is "fixed" as a result of another, independent mutation in a different gene. The amber suppressor is an example of such a mutation.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.