Fluorescence in situ hybridization (FISH) is a test that “maps” the genetic material in a person’s cells. This test can be used to visualize specific genes or portions of genes. FISH testing is done on breast cancer tissue removed during biopsy to determine whether the cells have extra copies of the HER2 gene. The more copies of the HER2 gene that are present, the more HER2 receptors the cells have. These HER2 receptors receive signals that stimulate the growth of breast cancer cells.
The FISH test results will tell you that the cancer is either “positive” or “negative” (a result sometimes reported as “zero”) for HER2.
There's more on that from this site: http://www.breastcancer.org/symptoms/testing/types/fish.jsp