Sometimes you hear from your supervisor- "You have to be proactive in research" or sometimes they ask you to be reactive. What does he mean?
Proactive activity means you are cautious about your future, you are planned to face problems in future, or you are prepared for something "bad". When you are saving money for facing troubles in future, it is said to be a proactive activity. So, proactive research means if you know a "situation" may occur in future, prepare yourself. To understand "situations" that may occur, you need to dig your problem and find all possible "situations" that may occur in future.
So, you are a proactive researcher when you analyze your problem and formulate solutions prior to some situations in future.
Reactive activity means a little bit carelessness: I will provide solutions when "situation" occurs or I will react when time comes. So, reactive researchers do not bother about future; they just do what they are meant to do and when they face problems, they try to find a solution.
Mostly, in my opinion, you need to have a blending characteristic to succeed in research: at times you need to proactive and sometimes being reactive will bring you success.