What is the significance of this quote from Cicero? "The first law of historiography is daring not to say anything false, and the second is not...

It should, first of all, be noted that the correct quote is actually:

"... the second is not refraining from saying anything true ..."

This changes the intent from something sarcastic or witty to a rather straightforward meaning.

Cicero was a Roman orator from the 1st century BCE, famous for being a massively influential politician, philosopher, and speaker.

The rest of the quote, when put in context, is about the difficulties of writing about historical content. He contrasts these first two rules—that one should say nothing false and leave out nothing true—with the expectations of the readers, who desire opinion, analysis, and explanation.

These last two are, for the historian, particularly difficult, since they must rely, on some level, on speculation. As such, including them is in direct violation of the first two rules, which demand nothing but facts be reported. Further, including opinions on events, as is often asked, conflicts with the last clause about being free from bias for or against.

This quote, then, is simply about the contradiction between the strict rules for the historiographer and what is actually demanded of them. It is impossible to fulfill both.