A concession refers to a situation in which you acknowledge the fact that an opposing point of view might have some merits. On the other hand, a rebuttal occurs when you are able to prove that the opposite opinion is flawed, making your argument stronger.
This is an example of a concession in the Declaration of Independence:
This quote is a concession because it accepts the argument of the opposing side. In this quote, the authors argue that prudent people agree that government should not be easily altered. They also argue that people often suffer greatly when their usual way of living is extremely changed. This argument seems to go against the purposes of the Revolution.
This is an example of a rebuttal:
In this passage we learn that, although the authors agree with the previous argument, they also believe that the Revolutionary War is not one of these cases, as the English government has been excessively cruel and abusive, which gives the revolutionaries the right to defend themselves.