MVCC — what do concurrent transactions see?
Answer out loud or in your head first, then compare.
Answer
MVCC keeps versions of rows, and transactions read data according to the visibility rules of their snapshot. So a plain read can use an older visible version while a new one is being written.
Example: one transaction updates a record, and a plain SELECT in another doesn't read its uncommitted change.
To be precise: MVCC doesn't mean there are no locks. Two writes to the same row can wait for each other. The isolation level affects when the view of the data is fixed.
Pitfall: MVCC doesn't automatically eliminate a “read, compute, write” race. Old versions need cleaning up by VACUUM.
This expands on the internals topic the author mentioned; it isn't a quote of a specific question.
How did it go? Be honest — “Not yet” puts the question on your review list.