Boss Nass is the leader of the Gungans. He's the head honcho, the big man on campus, the, um, late-career Marlon Brando of his people.
(Well, now we're just imagining what it would be like if Marlon Brando had played Boss Nass. That would have been awesome.)
Anyway, at the film's outset, Boss Nass doesn't care about the Naboo. He thinks the humans "tink day brains so big" and look down on the Gungans. Obi-Wan tries to warn him that the droid army will take control of the Gungans after occupying the rest of the planet.
As he says:
"You and the Naboo form a symbiont circle. What happens to one of you will affect the other."
But Boss Nass, full of scorn and dislike, shrugs away the warning. This scene also shows us how weak-minded the Gungan leader is as Qui-Gon effortlessly uses his Jedi mind trick on the guy. Seriously, Qui-Gon isn't even trying.
But Obi-Wan's prediction is accurate. The droid army does rout the Gungans from their underwater home. When the Naboo come to form an alliance to fight back, Boss Nass is initially distrusting, but then Padmé genuflects before him and begs his assistance.
Seeing he was wrong about the Naboo, Nass proclaims:
"Yousa no tinken yousa great den da Gungas? Me-e-esa like dis! Maybe wesa bein friends."
While he may not be intelligent, Boss Nass does prove he can be wise enough to realize his mistakes, and his peace with the Naboo proves necessary for the defeat of Trade Federation and the end of the Invasion of Naboo.
Once more, though: no discussion of Boss Nass would be complete without voicing the complaints about racism in The Phantom Menace:
"Jar Jar's, uh, people, happen to be ruled by a fat, jolly buffoon reminiscent of old stereotypes of African chieftains who boil missionaries in big pots." (Source)
And once more: what do you think? Is Boss Nass an offensive caricature? Are Jar Jar and Watto?