Reductio ad absurdum (reduction to the absurd) is a logical argument where you accept some proposed hypothesis for the sake of argument, arrive at some absurd result, and then conclude that the original hypothesis cannot be true since it leads to the incorrect result. The argument makes use of the law of non-contradiction which says that a law cannot be both true and false.
For example, if John says 'Raising tax rates always brings in more revenue for the government, I could respond with a reductio ad absurdum saying 'Lets imagine that the government raises taxes to 100%. Then people would have no incentive to work thus leading to less production and lower tax revenues.'
Though the reductio ad absurdum is a legitimate logical argument, you have to be aware of when it is being used against you to simply win a debate. Using the above example, perhaps John really only was arguing that taxes should be raised to a higher level than what they are currently at. To respond to my reductio ad absurdum, he needs to rework his original statement. Perhaps he might say that he misspoke with his original statement and he believes that raising tax rates an additional 10% would increase tax revenues.