Definition
A straw man fallacy replaces an opponent's actual position with a distorted, exaggerated, or simplified version of it, then argues against that easier target as if defeating it settled the original disagreement. The name comes from the image of building a scarecrow that only resembles a person, so it is easy to knock down.
Example
A debater argues that schools should offer more elective courses in the arts. An opponent responds: "So you think math and science don't matter at all?" Nobody claimed that. The opponent has swapped a specific, limited proposal for an extreme one that is far easier to reject.
Common mistakes
The most common version of this mistake is unintentional: paraphrasing an opponent's argument sloppily under time pressure and accidentally weakening it, without meaning to misrepresent anything. The fix is the same either way. Before you respond to an argument, restate it in a way your opponent would agree is fair. If you cannot do that, you are not ready to attack it, and a judge who is flowing carefully will notice the mismatch between what was said and what got answered.