Brisingr is from an ancient language that all living things used to speak. However, it was forgotten over time and went unspoken for eons in Alagaësia, until the elves brought it back from over the sea.— Brom
Brisingr was the object of "fire" in the Ancient Language (see also, Istalrí). The word is used by a magician for casting fire. Brisingr was used by Brom to light a fire when the tinder box would not work. It was also the first spell Eragon cast, although he originally thought it was a swear word that Brom used. He used it to kill two Urgals in Yazuac: the subsequent loss of energy almost knocked him unconscious.
Brom told Eragon that it is "not only a name for fire, it is the name for fire." Although, this is a very effective spell, it requires much energy to cast when one is not well acquainted with magic. It can be used in many different ways - lighting a lamp, as a weapon (e.g. a fireball), or as light itself (e.g. a werelight). It may even be used for cooking, as mentioned in Eldest.
Christopher Paolini has stated that brisingr was the first word of Old Norse that he found when investigating potential magic words and he liked it so much he decided to base the entire Ancient Language vocabulary on it. He also claims brisingr as his favorite Ancient Language word.
In the game, "Brisingr" is the only spell with an Old Norse name.
Eragon used Brisingr in many ways, from obliterating Urgals to helping destroy Durza.