The implication of the simile is that it links Daisy to a larger cultural perception that becomes the essence of Fitzgerald's work. In the second half of the simile, when Fitzgerald employs "the poor," the reader begins to see the class division which Daisy represents emerge. While Daisy is engaged in the social trappings of wealth and privilege, there is an entire other world which endures "the hot struggles" of being poor, or devoid of wealth. One reality which emerges from employing this image is the idea that America during the 1920s consisted of two Americas: One that had wealth and one that lacked it. The expression of this narrative defined how people lived during "the Jazz Age," so while the Daisys of the time period act "safe and proud," "gleaming like silver," there are more whose struggle is "hot" and brutal because of the primacy placed on wealth. This becomes a critical component in both Fitzgerald's work and his perceptions of the time period.
This is actually a difficult question to answer...not because the simile is a hard one, but because anyone who answers this question will have already read the book; we already know what Daisy is like. The question is far more asked of someone who is reading the book for the first time and still might not know that much about Daisy. Of course, by chapter 8 you should already have a pretty good idea about that....
Anyhow, let's take a look: the simile is comparing Daisy to silver. The author goes a step further and explains the simile by saying that she is "safe and proud and above the struggles of the poor." This gives you a pretty good idea of what Fitzgerald wanted you to understand about Daisy.
First, silver is a precious metal. It is not as expensive as gold, but it is not exactly cheap and it is a luxury item. The use of this metal in regard to the simile lets us know that Daisy is rich. Of course, we already knew that.
The more interesting part is the bit about being safe and above the struggles of the poor. This shows that Daisy's money is able to solve a lot of problems for her...it keeps her safe from many of the difficulties of survival that most people have to go through. Because she is "silver" and not some other base metal she also has the ability to remain proud above the unwashed "masses" of poor people. Most people in poverty have no chance of buying things made out of silver. Daisy stands apart from the crowds (or separates herself from it.)
Hope this helps!