What is a concession point?
A concession point is the part of your issue essay where you acknowledge another argument. The point is to show that you have thought about all sides of the issue in depth and that you can anticipate why someone would disagree with you. So, you are basically arguing against yourself in order to show that you understand the whole issue and can think about possible alternatives, even while showing that your argument is strongest!
How do I write the concession point?
The concession point should usually be in your third body paragraph (the fourth paragraph overall). It can take up a whole paragraph, but it doesn't need to — it can be just a few sentences. Check out how Chris incorporates his concession point in his sample issue essay. The concession point (in the fourth paragraph) makes its point without being very long.
You will notice that almost none of the prompts explicitly ask you to write a concession point, however they do typically hint that they want you to discuss what you might say to someone who disagrees with the point you want to make. Here's an example:
"Write a response in which you discuss the extent to which you agree or disagree with the statement and explain your reasoning for the position you take. In developing and supporting your position, you should consider ways in which the statement might or might not hold true and explain how these considerations shape your position."
Why is the concession point helpful?
Essays that deal with concession point do better because they are fully responding to the prompt! Showing that you have thought about the many facets of the issue and why someone would disagree with you shows critical thinking and broad-mindedness. And critical, broad-minded thinking is something that the GRE purports to test, and that graduate schools expect. :)