This is in reference to Simba's battle with the Outsiders, who were a group of lions that had been exiled from the Pride Lands by Simba's father, Mufasa. The Outsiders were led by Scar, Simba's uncle, who had been defeated and killed by Simba in the first film.
Zazu's words are meant to express his disapproval of Simba's decision to fight the Outsiders, as he believes that it is a reckless and dangerous move that could have serious consequences for the Pride Lands. He is also concerned about the safety of Simba's daughter, Kiara, who is in love with Kovu, a member of the Outsiders.
Zazu's words are also a reflection of the long-standing conflict between the Pride Lands and the Outsiders. The two groups have been at odds for generations, and Zazu believes that it is impossible for them to live together in peace. He fears that the Outsiders will eventually try to take over the Pride Lands, and he is determined to do everything he can to prevent that from happening.
It is important to note that Zazu's words are not necessarily true. It is possible that the Outsiders are not actually intent on taking over the Pride Lands, and it is also possible that they could learn to live together in peace with the Pride Landers. However, Zazu's fears are based on the history of conflict between the two groups, and he believes that it is better to be safe than sorry.