What should I (a professor) do if a student keeps answering other students' questions before I start explaining? I have a student who keeps answering other students' questions before I could even start explaining. What should I do in this case? Should I ask them to come meet with me? If so, how should I convey that it is not ok and it is not their job.
This would, I think, depend a lot on how the "answering" student goes about it. Just interrupting a request to the professor is disruptive, but some people do this sort of thing by reflex. But yes, a quiet word is in order. Ask them to meet with you. But decide first on what you would like them to do to correct the action. 

If the answers they give are generally correct then it is a different situation than if they are mostly irrelevant, I think. 

One thing that such a student fails to understand, of course, is that often the best answer is a minimal answer. An answer that lets students develop insight, not just get answers. Saying too much in answering a student question may be worse than saying nothing.

One option is just to require them to remain silent, but another is to ask (require) them to raise a hand before saying anything and first getting permission. But yet another, that might be useful, depending on the student, is to require them to write out an answer when they feel the need, rather than to blurt it out verbally. 

But the behavior can be a symptom of many things. Some are insecurities on the part of the student; a need for affirmation. Some would just be showing off for the professor. Without knowing what it is, it is difficult to formulate an exact solution. 

You have a right, of course, not to have your course disrupted by outbursts, but think a bit about whether the disruptor has needs that also need to be met. So, as part of your quiet word with them, ask first why they think it important to do that. Depending on your view of the student, the question might be direct, or you might want to be a bit more subtle about it. 

And sometimes it is actually useful to have students volunteer answers to the questions of others as long as you maintain control over the process. If one student is dominating a conversation, it might be counterproductive for others.