Laura Wingerd gave a fairly high-level introduction to shelving in her blog post:
You can cache your modified files on the server, without having to check them in as a versioned change. For teams, shelving makes code reviews and code handoffs possible. Individuals can use shelving for multi-tasking, code shunting, incremental snapshots, and safety nets.
The new commands are p4 shelve and p4 unshelve, and the blog post explains a bit of the workflow involved in using these commands to accomplish the various new scenarios.
I think it's going to take a bit of time to become comfortable with the new commands and how to use them, but I'm looking forward to getting this feature installed and available so I can start to learn more about it!