What does this mean? I hear it often, but not sure what its meaning is. I think it means to believe what they are saying without proof.
The derivation of "face value" in this sense most likely has to do with the phrase as used in talking about currency: the value of a coin or bill is directly apparent from the numbers printed on it.
So it means taking the meaning of someone's speech or actions directly. (w/o looking for hidden meanings as the other answerers have said)
Well, I couldn't take this question at face value and had to dig deeper (inspired by @sibbaldiopsis' answer).
There were two U.S. Supreme Court cases, Mandeville vs. Welch, 1820, and Lines vs. Sminth, 1851 that spoke of promissory notes needing to have the words "value received" written upon their faces. Could be some connection here.
The first instance of the exact phrase I could find in print was from Mason's Coin and Stamp Collectors' Magazine, 1870, referring specifically to the face value printed on postage stamps (in contrast to the value of the stamp to a collector).
The first instance of the term used for money I found was from Folsom's Logical Bookkeeping, 1873:
Currency too—the bank-note and greenback—has its face-value, which is its printed amount .
The first figurative use of the phrase I found, the most common way it is used today, was from O. F. Cook's 1891, Papers on Myriopoda:
I agree with Sibbaldiopsis above. My guess is that it goes back to Greek or Roman times. The rulers wanted to establish "fiat" currency rather than currency based on the commodity value of gold or silver. The ruler, whose face, a symbol of his authority, was on the coin, simply declared the value of the currency and expected merchants and consumers to trade with it at that value. In this way, debased currency could enable the ruler to increase the money supply without being tied to the intrinsic value of metals.
The currency we use today is "fiat" currency.