Snapshots allow you to save some scene properties to return to them later. This includes camera parameters and environment settings (like offset and intensity).
There are two reasons you may need snapshots:
To add a snapshot open the Snapshots panel at the left and click the “+” button at the top right corner of that panel. There’s also an item in the main menu that does the same. Snapshots are saved with the project, so you can use them later.
To activate a snapshot, simply double–click it in the list or use the context menu.
You can update a snapshot with the current scene state by right–clicking it and selecting Update with current settings item.
See here for how to use snapshots from scripting.