“He's got a great work ethic.”

You coach a high school baseball team. You're talking to the father of one of the players. This player tries really hard to improve in practices. You say:

He's got a great work ethic.

(someone) has got (something)

There's no difference in meaning between "has got ___" and "has ___". You could also say:

He has a great work ethic.

The choice between "has got" and "has" depends on where you're from. "Has got" is more common in British English. In American English, "has ___" is more common but "has got" is also used in more casual speech.

(someone) has a good work ethic

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.