Not awful, not great, just OK.
Our hero is Blu, a very rare bird kidnapped from his native habitat as a baby. Through a fortunate accident, he winds up in the loving hands of young Linda. Blu and Linda grow up together, he into an adult (but sheltered) bird, she into a shy (but intelligent and resourceful) librarian.
As it turns out, Blu is the only hope for his species to propagate, and Linda gets convinced by ornithologist Tulio to bring him down to Rio for some quality time with Jewel, a female of Blu's species, at Tulio's refuge. (The movie is a solid G-rating, so this is not the source of jokes.) But evildoers (human and avian) execute a plot to snatch Jewel and Blu for their own nefarious purposes. There's big money in rare birds! Soon the birds are trying to escape, while Tulio and Linda are giving chase. There are wacky antics, colorful characters, songs, dancing, and of course a climax staged during Rio's Carnival.
There are some laughs, but these days animations ostensibly for the kids have some pretty high bars, and Rio doesn't achieve the wonderfulness of Rango or Kung Fu Panda 2.)