A snapshot stores the state of the data area at the time of the snapshot.
Once you have made a snapshot, you can continue to work normally and make data changes. You can later decide whether you want to return to the state the data area was in at the time of the snapshot or accept all changes since the snapshot.
Database Administration, Snapshot of the Data Area
SQL Reference Manual, SNAPSHOTS
SAP Note 1423732 (FAQ: SAP MaxDB Snapshot)