Holism is the idea that all the properties of a given system (physical, biological, chemical, social, economic, mental, linguistic, etc.) cannot be determined or explained by its component parts alone. Instead, the system as a whole determines in an important way how the parts behave. Hence Holistic Medicine looks at lots of health factors to come up with a complete treatment. Unfortunately our society is in love with Reductionism, which is sometimes seen as the opposite of holism. Reductionism in science says that a complex system can be explained by reduction to its fundamental parts. For example, the processes of biology are reducible to chemistry and the laws of chemistry are explained by physics.
A reductionist view of ones problems, like being overweight, would have you focus on specific diet issues, like calories, or fat intake, or sugar intake, or too many carbs, basically whatever diet fad you're following. There is of course truth to each of these specific issues that reductionist processes lead you to. Obviously eating more food than your body requires will cause weight gain, but there is much more to the weight story.
In other words taking the reductionist path of problem solving leads to incomplete solutions.
The consequences can be major. Leaving an attribute of a problem unaddressed means eventually more problems down the road. The unaddressed attribute will continue festering, possibly it will remain a minor side issue that "isn't worth bothering with" or maybe it will grow and itself become a major problem.