It would be simplistic to say that people who get good grades study more. This answer doesn't account for the complexities of how we learn and are tested. When something is simplistic, it is overly simple.
When your mother tells you to just ignore a bully, you might find her advice simplistic. How can you just ignore his insults? She might also tell you that he probably doesn't have a good home life, or is jealous of you. These too will sound simplistic. The fact is, sometimes there is no simple answer to a problem, and all attempts to solve it quickly will be simplistic.