This is a really inaccurate description. Messages aren't indexed by timestamp in any meaningful way; that feature is currently under development. Messages don't need to be acknowledged, it's the client's responsibility to track what messages have been consumed. The server provides some facilities to make that easier, but ultimately clients can request whatever messages they want (repeating, skipping, whatever), as long as the messages haven't expired out of retention.

If you're actually interested in Kafka, just read the documentation, it's quite good.