The phrase take it with a grain of salt is an English idiom meaning, "Be skeptical about it." The "it" is usually a statement heard from someone else, like a factual claim or advice. It can also mean that while the statement is mostly true, it might include some error or it might not be true when applied in every circumstance; you will need to take care and use common sense when applying it.
Historically, the expression with a grain of salt comes from reports by the ancient Romans, such as Pliny the Elder in the Naturalis Historia, that adding a grain of salt to a certain recipe or to poisons could make you immune to poisons; see here for a little more information. Most English speakers today don't hear any reference to poisons when they hear the idiom, but the grain of salt vaguely suggests protection from something mildly dangerous in the thing that you are taking it with.
Quoting a little more of the context of your example helps show its meaning there:
Direct evidence about the ambiguous relationship of material and subjective well-being comes from studies of happiness that psychologists and other social scientists have finally started to pursue, after a long delay in which research on happiness was considered too soft for scientists to undertake. It is true that these surveys are based on self-reports and on verbal scales that might have different meanings depending on the culture and the language in which they are written. Thus, the results of culturally and methodologically circumscribed studies need to be taken with more than the usual grain of salt.
Here, the meaning of this variant of take it with a grain of salt is: "You should ordinarily be skeptical of results of psychological and sociological studies. But these studies merit even more than the usual skepticism, because they are based on self-reports and verbal scales that are open to a lot of ambiguity and misinterpretation."