It means to put the ship into service. A ship is "launched" when its hull is built and it can float. It is then "fitted" meaning that the controls, furnishings, weapons etc. Are added. It then may have it's sea trials to ensure that it meets the specification required and that all parts are working. Finally, it is "commissioned" meaning that it becomes part of the active fleet and is ready for use on missions. At the end of its life it will be "decommissioned" and taken out of use.