If you google User Story Mapping and Jeff Patton, his site will provide you with a lot of information. He even wrote a book on the topic.
To answer your question, User Story Mapping is a technique to visualize your Product Backlog and lay it out grouped by course grained features and associated Product Backlog Items (aka user stories). Some Scrum Masters or Product Owners may facilitate a user story mapping session to derive an initial Product Backlog and Minimal Viable Product (MVP). I have also seen it used to derive a higher level Roadmap. It is also a great way to make your Product Backlog transparent and to get stakeholder feedback, as it is easier for some to visualize the Product Backlog.
The features are high level and arranged horizontally in the order in which you will build your product. The features are called the 'backbone'. The more finer grained user stories are laid out vertically under the corresponding features, in priority order, and are called the skeleton.