Basically an argument where someone responds without actually addressing the core issue. Example:

Person A: I think we should spend less on fuel.

Person B: well that's dumb, because driving is more convenient than walking.

The second argument is a "strawman" because it doesn't actually address whether spending less on fuel is a good idea, but rather redirects to a different topic.

To clarify the metaphor, a literal strawman is a scarecrow or dummy. It's not real. Argumentatively your discursive opponent is setting up fake target to avoid engaging with the real issue you're trying to raise.