Stash
Save a stash
Repository › Save Stash… (⇧⌘H / Ctrl+Shift+H) or the Stash button on the toolbar, when there is something to stash. The dialog takes a message and two options: include untracked files and keep index (leave the staged changes staged).
Apply, pop and drop
Stashes are listed in the sidebar. A stash's context menu has Show, Apply, Pop (apply, then remove) and Drop…, which asks first.