"For Sale By Owner" means exactly that. The person who owns the home/property is selling it without the assistance of a real estate broker or realtor.
Depending on where you live, and what company you talk to you can end up paying anywhere from 2% to as much as 7% in commission to a real estate agent. That ends up being quite a bit of money, especially with todays housing prices. By selling your home on your own you can price your home more competitively since you're not paying a commission.