What is Chalitzah Signifigance? What does it accomplish, and why is it done?
If a man dies with no children, then his brother should marry the widow. If the brother chooses not to do so, then chalitza is a ceremony whereby the brother and the widow proclaim that he refuses to marry her; the widow removes his shoe and spits, and everyone acknowledges and proclaims accordingly.
So the simplest idea is that in the times of the Torah, the generally preferred approach was for him to marry her. If he chose not to, he would be publicly shamed. (Now in many cases such a marriage really would not be the best idea, but the Torah says do this procedure in all cases.) The Sforno notes the next topic in the Chumash is payment for embarrassing someone: we can shame people in specific cases for specific purposes, as warranted by the Torah; but that doesn't mean we can shame anyone willy-nilly. (And it's only because we're a society that cares about people's honor that shaming means anything.)
Now what is the significance of the shoe and the spitting? The spitting seems to be "ptooey, I'm disgusted that you didn't do the right thing." As for the shoe: