Who was Qassem Soleimani and why did the US target him?

Well, he's the most important military leader in Iran. The architect of their proxy warfare and terrorist network across the region, which made him one of most important antagonists of the United States. He specifically was targeted because of his involvement in ordering attacks against an American base in Iraq, led to the injury of US soldiers and the killing of one US military contractor. The fact that he was killed is a major escalation in the confrontation between the US and Iran that has been going on really since the US withdrew from the Iranian nuclear deal.