A pending changelist stores information about files you are working on. There are two kinds of pending changelists, the default changelist, and numbered pending changelists. Your workspace automatically has one default changelist. You can optionally create one or more numbered pending changelists in your workspace. To see a list of your pending changelists, use Go To -> Pending Changelists.
You can click on a changelist listed in the Pending Changelists page to go to its Edit Pending Changelist page. There you can edit the changelist, move files from it to another pending changelist, submit the changelist, or delete it.