User Story Mapping is an evolution of the traditional Agile backlog, made popular by Jeff Patton in 2008. It's an effective practice for creating lightweight release plans that can drive standard Agile delivery practices. At the end of user story mapping, you should have:
A backlog of scope items (captured as stories or simply feature titles) the team believes can be delivered in the planning window. This means some items will be placed out of scope
The backlog "sliced" into ~3 iterations, such that it forms the outline of plan
Enough detail in the first iteration of the plan to get started with the work
No one likes estimating work effort, but it's an important step in many environments. User Story Mapping presents an approach to estimation that many teams find much more tolerable than practices like planning poker or t-shirt sizing
Many projects have a natural progression of work from beginning, middle to end. This progression is often lost in a traditional flat backlog. User Story Mapping captures this information with a lightweight mechanism to plan out more than one iteration (i.e. an Agile release plan)
User Story Mapping presents backlog items in the context of the overall business process so you don't lose sight of "the big picture", a common pitfall in Scrum projects
Check out these great links which can help you dive a little deeper into running the User Story Mapping & Value Slicing practice with your team, customers or stakeholders.