What this line means is that Anthony Burns's case came to represent, in the minds of many in the North, the desire that slaves had to be free.
Anthony Burns was a slave who escaped to the North when he was 19 years old. One year later, he was captured in Boston and was tried under the Fugitive Slave Act. Burns was found to be a fugitive slave and was returned to the South by force. When this happened, Burns became a symbol. Abolitionists in the North used him as an example of the injustice of the Fugitive Slave Act. Burns came to represent the idea that it was horrible to take a person who had escaped from slavery and then send him back to that terrible condition.
This is why Anthony Burns became a "symbol of freedom" overnight.