Whenever a file is modified in a logical volume that has a snapshot running, a copy of the original, unmodified file is retained in the snapshot.
The snapshot allows you to “freeze” the filesystem at an arbitrary point in time while allowing the original logical volume to remain in use.
Although the word “snapshot” seems to infer that it is a full duplicate of the original logical volume, it is not. The snapshot only contains the originals of files that have changed since it was created.
The snapshot is especially useful when backing up a filesystem that is actively being used. When a logical volume snapshot is mounted and its contents copied to tape, to another drive, etc., what you get is an exact duplicate of the entire original logical volume at the moment the snapshot was created.