Google has answers to such simple questionWhat's difference between GSI enforcing and Permissive?
Permissive means SElinux is disabled. Some ROMs need to execute some commands in order to be able to boot and they need high level permissions. So SELinux needs to be permissive
Enforcing means SElinux is enabled.
So ROMs or apps don't get high level permissions. Therefore making your phone more "secure" to malicious apps