'Go Hard' means to do something with a lot of effort. 'Go smart' means to do something cleverly. So - the phrase means to do whatever you are doing with commitment and intelligence. I'm not sure about sentence structure - very few native speakers know the grammar of their native language. I also suspect that it isn't really a sentence - it is abbreviated. I think that the 'Go' here is an imperative verb, followed by a adverb.

I'm familiar with two expressions that are similar. We sometimes say "work smart", which means to make good use of your time at work. Instead of focusing on how many hours you work, focus on the quality of the time you work. If you work smart, you don't have to work vast numbers of hours to be effective. Another common expression is "work hard, play hard". This means that you should exert yourself both at work and at play. If you work hard, it's okay to play hard, and if you play hard, it's okay so long as you play hard. The idea is to have balance, both working and playing hard.