A snapshot is called 1 single frame from a video. If you take 1 frame the result is a photo-like image. Mostly in jpg or png format. Many camera manufacturers support the Snapshot format over a particular function API over CGI.
In Screenshottet snapshots ot of videos the generated image looks sometimes spongy / blurred.
You should always be trying to use the API function of the camera manufacturer, as the quality of the snapshot is higher.
Restreamer offers automatic snapshot. How this works is explained in our guides about snapshots.