As a noun, it refers to a state that you are full of energy, not tired... "还有精神“ means you still have energy and you are not tired, so you can continue to do the thing that you did before Also, it has other meaning as nouns and it can also act as adjective... It depends on context Hope my answer helps~

1.vigour ：活力；精力。e.g 精神饱满 、 我工作了一整天了，还是很有精神(i am still full of energy after working all day ) 2.lively ：活跃，有生气。e.g 这个小孩眼睛大大的，看起来很有精神(the child looks like lively with her big eyes) 3.spirit、mind：意识、思维、神志等。e.g 我们靠团队精神赢得了比赛(we won the match because of teamwork) 我们需要民主精神（we need the democratic spirit）

In addition to the other answers, you can also regard it as a energetic state of mind. For example, if you had a football match and making good decisions that day whilst playing and energetic, then 精神很好. Or taking a exam and that day the difficult questions seem straightforward and you answer clearly, then 精神很好.

这个词的含义其实是找不到对应的英文的。

最好提供一下上下文