A snapshot stores the state of your work at an arbitrary point in time, much like a freeze frame.
In contrast to reverting edits step-by-step using the History panel, snapshots let you instantly restore your work to an explicitly defined stage.
You might do this in advance of carrying out a sequence of complex operations to allow yourself to step back to a specific point in time if things don't go to plan.
Multiple snapshots can be created. So, if you're experimenting with different design ideas, you can also create a different snapshot for each one to help you decide which idea you prefer, all while working in a single document.
If needed, you can create a new document from a snapshot.
The Snapshots panel is available in the Photo Persona and the Develop Persona.
In the Develop Persona, the panel is visible by default in the right studio.
In the Photo Persona, the panel is hidden by default. It can be switched on via View>Studio.
In the Photo Persona, it can be used in conjunction with the Undo Brush Tool to paint back to a chosen snapshot. Select Set undo brush source (the camera icon) on the required snapshot's row.