Definition: When someone judges the decision of someone in the past in the light of knowledge that was not available at the time. In other words, viewing their past actions through the lense of new information, assuming that they had the same perspective or information during their time.
Example: You shouldn't have taken the freeway! Then you could have avoided the crash and not have been late to dinner.
In reality, they weren't aware that there was a crash, and thus didn't have the information necessary to make a smarter choice.
Example: I can't believe that people in the 15th through 19th centuries thought that tobacco was good for you! If I lived then, I would not have smoked at all.
While people during this period thought it was either a panacea, anesthetic, or good for teeth, they were believing this without the knowledge of the smoking- lung cancer link, later discovered in the 1940s and 50s.
Claim X was made in the past
Those who made the claim didn't know Y, which wasn't available to them at the time.
Thus, this was a foolish claim