The way I recommend thinking about snapshots is to take one any time you feel you’ve achieved a good result with an image but plan to go further with your adjustments. This will give you the ability to go back to that point at any time by simply clicking the name of the snapshot you created.
To add a snapshot, adjust the image to the point you’re happy with it (or the point where you want to capture a snapshot) and then click the "plus" button to the right of the Snapshot label on the left panel. A new snapshot called "Untitled Snapshot" will be created, with the text in edit mode. Simply start typing to apply a more meaningful name to the image. Ideally you want to apply a name that will indicate to you immediately what the image will look like if you choose that snapshot.
If at any time you decide you don’t need a snapshot you have created for an image, simply select it and then click the “minus” button to the right of the Snapshots label on the left panel.
Note: Snapshots don’t replace the History option discussed next, but rather supplement it by providing you with a way to encapsulate multiple adjustments into a single history state as a snapshot.
For more information about Photoshop Lightroom, see the following: