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...
Take it with a grain of salt - I always thought of this as an all encompassing saying meaning "consider your sources", "It may not be wholly true", "Don't take is as gospel", "Don't believe...
To take something with a "grain of salt" is to not take something too seriously. For example, I take everything that politicians say with a grain of salt, because history shows us that politicians...