An isolated VLAN is a secondary VLAN whose distinctive characteristic is that all hosts connected to its ports are isolated at Layer 2. A private VLAN has only one isolated VLAN. If the swPrivateVlanIsolatedVlanIndex has a value of zero, this indicates there are no associated isolated VLANs in the private VLAN domain. Otherwise, it is the associated isolated VLAN ID. A community VLAN is a secondary VLAN that is associated with a group of ports that connects to a certain 'community' of end devices with mutual trust relationships. There can be multiple distinct community VLANs in a private VLAN domain.
Sources for this description
D-Link official switch MIBsD-Link MIB copies (LibreNMS/Observium Community)