The Planned cache list is a list of caches which you are planning to complete in the future.
If you have a mate at GCA then by visiting your mates page you can see whether you have any caches that you are planning to do that also appear on their planned caches list. This way you can get together and tackle the joinly planned caches together.
A cache can be added to this list by going to that caches particular page and clicking on 'Plan' in the sidebar on the right.
You can view/edit your Planned cache list by going to the My tab and selecting 'Planned'