Kotoamatsukami refers to the five deities that appeared at the Creation according to the Kojiki (The Records of Ancient Matters).
The opening of the first volume of Kojiki says that, at the creation of heaven and earth, each of the following three deities (The Three Gods of Creation) became 'Hitorigami' (a deity without having Meotogami as a pair) at Takamanohara (plain of high heaven), and immediately hid themselves.
Ame no Minakanushi no Kami
Takamimushuhi no Kami
Kamimushuhi no Kami
Then, the following two deities appeared, when the land was formed and became like a jellyfish floating in the sea. These two deities also hid themselves as Hitorigami.
These five deities are called 'Kotoamatsukami' and are special even among Amatsukami (gods of heaven). After Kotoamatsukami, seven generations of the celestial gods appeared.
Kotoamatsukami, which because they came into the world during the earliest period, their divinity receive special treatment, and are supposed to surpass Amaterasu Omikami (the Sun Goddess). This is indeed inconvenient for the Emperors and therefore the Imperial Court, who want to authorize their own crown by regarding Amaterasu Omikami as the top god of Takamanohara and claiming themselves as its descendants. It's conceivable that Kotoamatsukami were hidden in the process of editing Kojiki and Nihonshoki (Chronicles of Japan) in order to resolve this contradiction. Thus, they were considered to be "special," but having no influence even though they exit.
In fact, Izanaki no Mikoto hid himself after throwing out Susanoo no Mikoto to Yominokuni (Hades), and Izanami no Mikoto died before Amaterasu Omikami was born, which poses no threat to Amaterasu Omikami's position as the top god.
Table of emergence
This is a table of emergence of each book. They are in the order until the five deities are included in Kojiki, and Kuninotokotachi no Mikoto (eternal god of the land) and the later deities are omitted.