at-least-once delivery means that for each message handed to the mechanism potentially multiple attempts are made at delivering it, such that at least one succeeds; again, in more casual terms this means that messages may be duplicated but not lost.

What is exactly once semantics?

Exactly-once semantics.

As its name suggests, exactly-once semantics means that each message is delivered precisely once. The message can neither be lost nor delivered twice (or more times). Exactly-once is by far the most dependable message delivery guarantee. It's also the hardest to achieve.

What does at least once delivery mean?

At-least once as the name suggests, message will be delivered atleast once. There is high chance that message will be delivered again as duplicate.