Most changes made within the lighttable are recorded and can be reverted to a previous state. This includes modifications to color labels, ratings, geo-localization, tags, metadata, orientation, copy/paste of history, image duplication, or application of a style. The undo/redo facility is unlimited in the number of steps while in the lighttable, but it is reset each time you switch to a different view.
Press Ctrl+Z to undo the last modification and Ctrl+Y to redo the last undone one (if any). These are also available from the global menu, under Edit 🡒 Undo / Redo.
The darkroom has its own, independent undo/redo for image-editing actions; see the history of changes.