It's a method to detect specific DNA sequence orgene expression. YOu use gel electrophoresis to separate the denatured DNA sequence. Transfer the DNA onto a cellulose paper for probing. Then you use a probe that can specifically bind to a sequence(or gene) you want to find. Usually radioactive labelled or chemiluminescence labelled for signal detection later. Signal detected---> means there is gene you want to find in the DNA mixture applied to gel in the beginning. Usually southern blotting is used for checking gene expression.