It is used to ask about someone if they want to do something. For example A: 우리 오늘 클럽 갈건데 We're going clubbing tonight. 생각있어? How do you think(=Do you want to go clubbing with us)? B: 아니 나 좀 피곤해. No, I'm a bit tired.

I have a thought (or you or anyone since the subject is not specified). 생각 is a thought, from the verb to think which is 생각하다. 있어 means to be present, 있다. 나는 그런 생각 있어. I have that kind of thought/thinking/idea 다른 생각 있어? Do you have another thought/idea?