A person with "a good work ethic" works very hard. This can be for school, sports, or a job.
An "ethic" is a philosophy or set of rules that a person decides to follow. So when you say that someone "has a good work ethic", it really means that they have a good attitude or philosophy about work. They believe that it's important to work hard.
Having "a good work ethic" is a good thing, so you shouldn't use this phrase to talk about someone who works too much. To talk about a person that works too much, say:
She's a workaholic.