In IKEv2 terminology, the first phase consists of the (usually four) messages that create the IKEv2 SA and the first associated IPsec Security Association (known as a child SA).

The four messages

IKE_SA_INIT: IPSec-Connection Initialisation
IKE_AUTH: Authenticaton of Communication counterparts
CREATE_CHILD_SA: Key change and invoking additional CHILD_SA
INFORMATIONAL: Further Exchange of Information

The IKEv2 SA is a protected tunnel that allows for subsequent messages to be sent cryptographically protected; this allows the sending of encrypted and authenticated notification messages, reliable dead-peer detection, and creation of additional child SAs.