Page Designer is a drag and drop style interface for creating and editing rich content inside Commerce Cloud without the need for HTML knowledge or developer tooling.
Content editing tools in Commerce B2C have always been lacklustre in comparison to proper CMSs (understandably) and unless you knew HTML, you needed external content tools to be enabled to do a lot of content work. Page Designer starts to change that and gives the platform good core content tools.
It's in pilot now, but it was demo'd at Dreamforce, see minutes 22 to 29 of this video. Once available, you'll want to discuss with your SI to give you a set of templates and components to help you power your page designs.