What is port membership type

I have a core switch and distribution switch where all the ports are configured as a trunk including core switch am trying to connect and configure the ap but it’s not communicating with vlan gateway even tried to ping from core switch response as a packet lost do I need to configure all the ports on trunk is it mandatory

During the time of login on Cisco sg-350-28p on a vlan management all port are set to trunk and membership as untagged it confused me

Is it refers to send all the traffic to native vlan

If it’s configure so the switch will higher cpu utilization?