Temporary IPv6 address
Lets image we only have 1 IPv6 address for our client which for example has been automatically generated by SLAAC (which contains the MAC-Address of the network card).
If this IPv6 address is being used to connect to the internet it would be very easy for tracking tools to identify you as an individual and create a profile.
Thats why random temporary IPv6 addresses are being generated to establish connections to the internet. Since these temporary IPv6 addresses are being deleted and regenerated periodically (depends on the system how often) its pretty hard for tracking tools to create a profile just on an IP address basis.
Secured IPv6 address
ATTENTION: I haven’t verified this information, therefore its just my speculation!
Secured IPv6 addresses keep unique for one interface inside a specific network.
For example you will get the same secured IPv6 address in your home network or you company network to access e.g. a special network share.
Currently this “secured” IPv6 feature is only visible by default on MacOS (June 2019)