EDIT in response to comment. I Am Not A Lawyer. If you want a legal explanation about what each section in the GPL means, and how they interact, contact a lawyer in your jurisdiction. The answer below explains the overall meaning of the GPL as explained in the FAQ. My understanding is that this is correct in many jurisdictions (i.e. it has been tested in court).
Technically the GPL does not prohibit selling software. But it does require that any purchaser must be allowed to distribute the software to anyone else, free.
Clearly this requirement effectively prohibits many business models for selling software for a living! You may be able to make some money by selling other things along with the software: support, warranties, training, documentation, consultancy, hardware...
If you own the copyright on the software, you can dual-license it: sell a paid version under different license terms.