Auto Discovery VPN are a type of IPSEC VPN using extensions set out in RFC7018 “Auto-Discovery VPN Problem Statement and Requirements”.
It is used when you have many connections and full mesh is not possible all the time as for every connection it needs to be added in rest of them as well, but here ADVPN allows a minimum amount of configuration per branch but still allows direct IPsec connections to be made between every branch.

Devices that support the this extension send an ADVPN_SUPPORTED message in the IKEv2 Notify payload including its capability information and the ADVPN version number during the initial IKE exchange. A device that supports ADVPN can act as either a shortcut suggester or a shortcut partner, but not both.