To me, Olaf symbolizes the love between Elsa and Anna. That’s because the time when most of Anna’s happy memories of her childhood with Elsa were altered, especially the ones that involved magic, Olaf is that familiar snowman that has remained. He serves as that one special link between them, mainly when they grew apart.
It is very important to point out that it was Olaf who helped Anna figure out what true love really means, and this had saved her and Elsa’s lives at the end. It was true love that prevented Hans from killing Elsa. It was true love that unfroze Anna back to life. And it was love that enabled Elsa to thaw the ice. If Anna had died, Elsa would also be good as dead, because her world revolves around Anna. As Anna had put it, they “only have each other.”