There are a number of possible answers to this question. I would suggest that you look at your textbook and/or your class notes to see what answer your instructor is looking for.
Perhaps the most likely answer is that the yellow press stoked the American public’s desire for aggressive foreign policy, particularly in the time of the Spanish-American War. The yellow press was very sensationalistic and was not terribly concerned with facts. These newspapers wanted to sell papers more than they wanted to inform the public in a responsible manner. Therefore, they exaggerated and even invented Spanish atrocities because they knew such stories would sell papers. These exaggerations and inventions helped whip up the American public’s desire for war. This is the effect of the yellow press that is most often discussed in textbooks.
However, you could also say that the yellow press had other impacts on the American public. For example, you could argue that the yellow press whetted Americans’ appetite for sensationalism and thereby helped to coarsen American culture and make it more ignorant. Because the yellow journalists were so eager to sell papers, they gave readers the sorts of scandalous and lurid stories they wanted instead of stories that would have done more to inform them about important current events. By doing so, we can argue, they “dumbed down” the American public.
On the other hand, you could say that the yellow press affected the American public by encouraging reading and attention to public affairs. The yellow press pushed very hard to get people to read newspapers. By doing so, they encouraged people to get interested in newspapers and reading where they might not have been before.
All of these are possible answers to this question. Please check your course materials to see which one your instructor expects to see.