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...

This quotation is taken from Cicero's De Oratoreand reflects a debate in antiquity about the purpose, style, and methods of historical writing. For Cicero, as for many ancient authors, history was considered a branch of rhetoric. The purpose of history was not simply to be a record of facts, but to teach moral lessons. This led to significant debates about how much a historian should simply stick to bare known facts versus the degree...

This quotation is taken from Cicero's De Oratore and reflects a debate in antiquity about the purpose, style, and methods of historical writing. For Cicero, as for many ancient authors, history was considered a branch of rhetoric. The purpose of history was not simply to be a record of facts, but to teach moral lessons. This led to significant debates about how much a historian should simply stick to bare known facts versus the degree to which it was necessary to include commentary and even imaginative reconstruction. 

Thucydides, for example, famously stated in his history, The Peloponnesian War:

With reference to the speeches in this history ...  it was ... difficult to carry them word for word in one's memory, so my habit has been to make the speakers say what was in my opinion demanded of them by the various occasions, of course adhering as closely as possible to the general sense of what they really said.

Cicero seems to be arguing, contrary to Thucydides, that rather than dramatically reconstruct speeches or events, the historian should refrain for doing so even when the reconstructed speeches may be relatively close to the originals. The debate here focuses on the question not so much of commentary versus facts but of readability versus accuracy.

We should also note that De Oratore is itself written in the form of a dialogue in which Cicero uses real characters and invents for them imaginary speeches; thus we might not want to take the argument of one character in the dialogue as a reflection of Cicero's actual views.