It is like "hey, go back to your sens" or pull it together . For example if you are sleepy during class and the teacher was looking at you your friend will whisper in your ear "야, 정신차려" Or you have a presentation and you start panicking your friend will tell you 정신차려. 할 수 있잖아. Pull yourself together, you know you can do it. Watch korean drama they use that expression pretty much. You will understand it more when you listen to it in different situations.