In common parlance, if something is to be taken with a grain of salt, it means that a copious measure of skepticism should be applied regarding a claim; that it should not be blindly accepted and believed without any doubt or reservation. According to the Oxford English Dictionary "to take 'it' with a grain of salt" means "to accept a thing less than fully". It dates this usage back to 1647.
The phrase comes from Pliny the Elder's Naturalis Historia, regarding the discovery of a recipe for an antidote to a poison. In the antidote, one of the ingredients was a grain of salt. Threats involving the poison were thus to be taken "with a grain of salt" and therefore less seriously. An alternative account says that the Roman general believed he could make himself immune to poison by ingesting small amounts of various poisons, and he took this treatment with a grain of salt to help him swallow the poison. In this version, the salt is not the antidote, it was taken merely to assist in swallowing the poison.