You should take what you hear and evaluate it on your own, don't
take it for being the truth or correct. The phrase is usually used
when a person it giving you the 'low down' on what another person
has told you. It is a warning that what that person has said, or
may say, is not necessarily correct and accurate.
My grandmother used to say this to me all the time. Basically, it
means to be skeptical or to question something that someone has
told you. For example, if someone has a tendency to exaggerate,
you'll want to take what they have said with a grain or pinch of
salt. Answers.com says that the expression is a translation of the
Latin cum grano salis
, which Pliny used in describing
Pompey's discovery of an antidote for poison (to be taken with a
grain of salt).
To take 'with a grain of salt' means to take with a heavy dose of
skepticism, caution and suspicion.
The saying came from the old cure for poison - a pinch of salt.
Salt was said to have healing properties, so to eat a meal 'with a
pinch of salt' meant that you suspected the meal of being