句意：那个人说话的口气好像他是认真的。

mean business“是当真的，不是开玩笑的，严肃的；正经的”是惯用短语，常用于口语。又如：

Her purposeful stare showed she meant business.从她坚定的眼神可知她是认真的。《简明英汉词典》

This time he wasn't fooling; he meant business. 这次他可不是闹着玩的, 他说到做到。《现代汉英综合大词典》

By sending their top people to meet our leaders at the summit conference，they really showed they meant business．在这次最高级会议上，他们派出了他们的首领同我们的领袖见面，这表明他们是郑重其事的。《英语常用短语词典》