The title of the film is a biblical reference, Matthew 5:13: 'You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot.'
In regard to his photograph of a gorilla putting its finger in its mouth, Sebastião Salgado states in the film that it is recognizing its own image for the first time after seeing the reflection of the lens. However, studies have shown that gorillas probably can't recognize themselves in mirrors.
The movie title "The Salt of the Earth" is also a reference to the photographer's last name. Salgado is Portuguese for salty. If you add salt to something, it becomes "salgado". This can be interpreted in a wider way as the contribution that Salgado has given to the Earth (planet) or in a more literal way as the amazing change that he and the Salgado family has made in to their land, bringing back the native Rainforest to the Instituto Terra (the Earth Institute).