If you wish to start a new game with all the skills and items from your previous playthrough, New Game+ DLC lets you do that.
If you have the base version of the Witcher 3, you need to download the DLC - NEW GAME PLUS on the platform you play the game on (instruction on how to obtain the DLCs). If you own Complete Edition/Game of the Year version of The Witcher 3, the DLC is already embedded into it and cannot be installed separately.
You can start New Game+ from the Main Menu after finishing the main storyline. Note that you cannot start New Game+ on a gamesave from an existing New Game+ file, you will need to start a regular, new game to begin a new journey with Geralt.