A "straw man argument" is a debating technique in which you say that your opponent believes something that he doesn't actually believe, and then spend your time fighting that point. Suppose that two people are debating which is better: vanilla ice cream or chocolate ice cream. The person who likes chocolate might say "Anyone who doesn't like dark ice cream must hate black people. Personally, I think racism is horrible," and then spend the rest of the debate talking about the evils of racism. The person who likes vanilla might say, "Vanilla is grown sustainably in the rain forest. If you don't like vanilla, that means that you want cattle ranchers to burn down the rain forest. Personally, I think it's bad to burn down the rainforest." And so the two will continue to talk about racism and environmentalism, and neither will listen to the reason that the other person ACTUALLY likes chocolate ice cream or vanilla ice cream--of course, they both actually just like their favorite ice cream because they think it tastes good.

Knocking down straw men involves making weak arguments against your own point in the context of a debate and then discrediting them. The term is used to draw attention to use of the technique by one's opponent.

thanks john

It's a deceptive debating technique: 1. First you introduce an argument against you that is easy to defeat, and you pretend that your opponent is making this argument (this is "setting up" the straw man). 2. Then you argue against this fake argument instead of your opponent's ACTUAL argument (this is "knocking down" the straw man).