Believers are humans, primarily children, who believe in the Guardians. It is their belief that fuels the Guardians' powers and magic, as well as their existence. They are also called "Lights" which symbolize their status as a believer.
Since Believers are primarily children, it is the Guardians' solemn duty and pledge to protect the Children of the World.
If a Believer loses faith in any particular Guardian and no longers believes, they lose the ability to see or hear that Guardian, even if he or she is right in front of them.
But so long as even one child believes, the Guardians will exist, however weakened, and Believers can help prove to those who lose faith that the Guardians are real, and restore their Belief, thereby restoring the Guardians' powers.
A child who believes is shown on The Globe of Belief in North's workshop, as a light, and in North's own words, "Good or bad, naughty or nice, [the Guardians] protect [the children]."
Believers are the primary source of the Guardians' power and existence. They are able to interact with them as well with other magical creatures and supernatural beings like the Boogeyman, Pitch Black, who is not a Guardian, the Fairies, Elves, Yetis, etc.
A believer has the ability to turn a living Nightmare into a Dreamsand manifestation of their own dreams.
In Nicholas St. North and the Battle of the Nightmare King, after Ombric fought the possessed bear, he was able to get better with the help of belief from the children of Santoff Claussen.
This is can also be seen in Rise of the Guardians, when the Guardians got their full energy back after Jamie and his friends were able to turn the Nightmares back into dreams. They can also restore a Guardian that has been defeated or destroyed.
Ombric has taught the children of Santoff Claussen how to perform magic with their belief and is mention that the first spell they learned and Ombric's more powerful one is "I believe, I believe, I believe...".