Idempotence of an operation means that if you perform it a second (or third, etc) time it won't do anything. The "action" all happens the first time and further goes at it do nothing. Eg. switching a light switch on could be seen as "idempotent" in a sense. You can press the bottom edge of the switch again but it's not going to click again and the light isn't going to become any more on.

The concept originates in maths, where it's functions that can be idempotent. The canonical example is projection operators: if you project a vector onto a subspace and then apply that same projection operator again you get the same vector again. In computing the term is sometimes used fairly loosely/analogistically like in the light switch example above. Sometimes, though, there is a mathematical function involved that is idempotent in the mathematical sense.

A form of idempotence is implied in "retries ... can't produce duplicate work" in the quote, but it isn't the whole story. Atomicity, for example, is also implied by the whole quote: the idea that an operation always either completes in its entirety or doesn't happen at all. That's independent of idempotence.