Undo takes back your last change, and Redo puts it back again. Keep clicking to go further: Labeljoy remembers your last 64 changes to each document.

Undo and redo
The buttons	Undo and Redo are on the right of the toolbar. Rest the mouse on one to see which change it'll undo or redo. They're dimmed when there's nothing to undo or redo.
Ctrl+Z	Undo.
Ctrl+Y, Ctrl+Shift+Z	Redo.

What counts as one change. Pretty much anything you do to the document: adding, deleting, moving or resizing an element, editing its properties, changing the layout, connecting data. A drag in the work area is one change however far it goes, and so is moving an element with the arrow keys. Undo something and then make a new change, and the changes you undid can't be redone any more.

While you're typing in a box of the tools panel, Ctrl+Z and Ctrl+Y work on the text you're typing, as they do anywhere else.

Each document has its own history. Undo works on the document on screen, and never touches the others. Saving doesn't clear the history, so you can still undo changes you made before you saved. Closing the document does: when you open it again, there's nothing to undo.

Changes made by Joy. Everything Joy, the AI assistant changes in one answer is a single change, so one Undo takes all of it back. You can also ask Joy to undo it.

On a document protected against editing Undo and Redo are dimmed, as there's nothing you can change.