What is a "snapshot"?

[Irl]

I was intrigued by the update notes about v3 regarding snapshots. However, there's nothing in the user manual about such things. Is there a new user manual I can download? How do I tell whether I have the newest manual? And if it's not there, could maybe some kind of simple description be posted here until it is?

[dnanian]

A snapshot is an APFS feature that basically represents a frozen moment in time of the entire file system.