A rental offer "without skipper" is a charter where you get the yacht alone: you will be steering it. Do not forget to specify your nautical experience and add your yacht license, in some cases, to reassure the boat owner. You will be responsible of the yacht during the entire length of the charter.
A rental offer "with skipper" is a charter with a professional skipper or the boat owner on board. They will be with you during your charter and will be in charge of steering the yacht.
A rental offer "with or without skipper" means that the yacht is available for charter with or without a skipper, both options are possible. It depends on your preference, and your nautical experience. Read more about What is the difference between rental offer with or without a skipper?