When people say that something should be taken with a grain of salt, they mean that it is a very good idea to introduce a measure of skepticism into one's evaluation of a situation. The saying is a reminder that people often wear blinders and do not think things through thoroughly, especially when they sound too good to be true. This idiom has ancient roots, thanks to the long intertwined history of humans and salt, a truly useful spice.
People have been telling each other that things should be taken with a grain of salt in English since at least the 1600s, but the origins of the phrase are much older. In fact, the first person to suggest it was Pliny the Elder, writing in the first century CE. Pliny included a measure of salt in an antidote for poison, and added that people should take threats of poisoning cum grano salis, "with a grain of salt," in a reference to this recipe. Salt actually is effective against some poisons, although the ingestion of large amounts can make a person feel rather ill.
When someone takes a threat with a grain of salt, as Pliny advised, it means that he or she does not take the threat entirely seriously, because he or she has information which makes the threat less scary or effective. Lots of things can seem threatening and overwhelming, but a person can find ways to make them more manageable, whether the threat is the potential loss of a home or a bad grade. In this sense, the proverb is a reminder to calm down and look around for common solutions when are presented with a problem.
A dose of skepticism can also be really useful when evaluating a situation that looks ideal. For example, if someone offers to send another person on an all expenses paid trip to a tropical locale, he or she may want dig deeper to find the hidden catch. If such an offer is taken with a grain of salt, the potential problems will often be revealed. In this sense, the saying reminds people not to accept things at face value, but to dig for more information to get a complete picture.
Some people are criticized for being too skeptical, but there's nothing wrong with remembering that things should be considered carefully. It can keep people out of trouble, and over time someone may even become known as being a bit sharper than other people, because he or she takes the time to fully consider something before responding to it. A measured response to an issue is often better than a hasty one.