A snapshot is a graphical representation of the state (type and data) of a data object at a certain point in time that you can use when you build stories. The snapshot you take is a copy of the state. This means that the state of the snapshot does not change when the state of the corresponding data object gets updated.
Snapshots capture individual objects on a sheet during the analysis process. They store the visualization and data as you see it at that time enabling you to use them at a later point in time to tell a story. Each snapshot contains a bookmark back to the original context so that you quickly get access to the live data.