It is difficult to overestimate

Could someone tell me the meaning of this following sentence:

"The importance of A is difficult to overestimate."

I am uncertain whether it means A is really important or means A is unimportant.

Taking it literally, it means that if one were to estimate the importance of A (for example, as if to assign a value to the importance or to give the importance a relative priority) they would have difficulty coming up with an estimate that exceeds its actual importance.

Taking it figuratively, it means that there is probably nothing more important than A, but if there was, it would be difficult to identify what is more important.

The expression contains a form of litotes, though not in the strictest sense of emphasizing a positive in negative terms (e.g., "A prophet is not without honor except in his own country and among his own family," which means "A prophet has honor everywhere but in his hometown").

I am assuming that the letter A stands for something, such as "preparing to give a speech before a hostile crowd."

Given that scenario, then, we could say without fear of contradiction that the more--and the more intelligently--one prepares a speech to be given before a hostile audience, the better one's chances of achieving no small measure of success (sorry for the "real" litotes!).

To say it's difficult to overestimate is to say it's easy to underestimate. Consequently, in the speech scenario, if the speaker underestimates the amount and quality of preparation s/he needs to invest in developing the speech, s/he is saying in effect that the speech is not that important.

Therefore, one is better off preparing too much than too little. Hence, the speaker would not be off the mark by investing "too much" time (i.e., overestimating prep time) because "too much" might prove to be the key to a successful speech, whereas underestimating the amount of prep time needed could spell disaster!

It's a tricky concept, but once you "get it, you've got it." Hang in there!

Taking it literally, it means that A is of the utmost importance and it is hard to evaluate its exact worth because of this.