It’s a Chinese modal particle，here used to express suggest and persuasion.

啊, 呀，呢，咯，etc, these words don't have any specific meanings. When they are in the end of an affirmative sentence, their function is just to make the sentence has a kinda of emotion. For this semtence "work hard!" 好好工作啊。啊here makes the whole sentence feel more gentle, less order feeling. You can feel it by youself.