mongodb – WiredTiger serves data. What exactly is the in-memory data?

I’m having a hard time understanding this paragraph

At the start of an operation, WiredTiger provides a point-in-time snapshot of the data to the operation. A snapshot presents a consistent view of the in-memory data.

  1. What is a point-in-time snapshot?

Maybe I’m confusing with the wording and any write operation is written to a this in-memory file/snapshot with a specific timestamp, sort of?