[Sonhex]

Originally Posted by Neki Ecko

Spoiler for Grandparents' Answer:

To tell him that he hold the key to figure out what destiny that Zuko can take, to be like Roku and maintain balance or be like the Fire Lord and have power over all Tribes. Plus he is next in-line for the throne, so Uncle is telling him, that he can decide the fate not only for the Fire Nation but all the Tribes.

...

Incidentally I think Zuko is only next in line as long as he's in favour with the Fire Lord. I guess that's Azula big plan, engineer her brother's failure (it's only a matter when not if) and then take the throne when her father pops his clogs. I'm pretty sure her current plot against Zuko is engineered precisely for this goal. She maybe a monster but she is a supreme tactician. I'm only sorry we don't get more development for her, even the beach episode didn't really shed much light on the source of her evil beside some sibling jealousy...