There are two actual ways of measuring the time of Arda’s history, split between the holy society of the Ainur’s reckonings and those of Arda’s own mortal races. It’s during the Ainulindalë that the initial split between Eru Ilúvatar and Melkor - one of the Valar who sought not harmony with his creator but power akin to it - begins as Melkor attempts to bring discord to Ilúvatar and the rest of the Ainur, corrupting many of the Maiar to his cause in the process. The Valar and Maiar then took to Arda (granted with Ilúvatar’s vision for what the future of this creation would be) to guide it and its beings along their divine plans - but it’s not all happy dealings. In The Silmarillion, this process is described like a heavenly jam session - Ilúvatar would sing to his Ainur the concepts and ideas he would have for creation itself, and they, in turn, would take those themes and develop them into harmonies that would eventually culminate with the creation of Arda itself.
It’s Ilúvatar who sings the song of creation to make their most important agents, the Ainur - the fourteen angelic Valar and their own servants - and the primordial spirits called the Maiar, that Eru Ilúvatar actually leaves most of the busywork of, y’know, the creation myth, to. Before the very concepts of time and matter exist, the first being brought into existence is Lord of the Rings’ answer to a divine power: Eru Ilúvatar. The Ainulindalë, as it’s known in the Elven language Quenya, is the creation myth about the very foundation of not just Arda itself, but the mythical beings that would shape it for generations to come.