You take someone's argument and turn it into something they didn't actually say, so then you can attack that fictional thing because that is easier than countering what they actually said.

Person A says something simple, like they wish income tax rates were lower.

Person B responds by saying 'oh so you want people to die then because there will be less money for social programs? How you could be such a monster?'

Person B created a strawman out of Person A's simple statement that they wished for lower income taxes and attacked the strawman instead. When the reality might be that person A wants income taxes lowered but for there to be higher corporate taxes and thus no 'people dying from the reduction of social programs.'