Issue while installing sys-vpn!

I am trying to install sys-vpn as a gateway according to this guide

for this step :
Create directory /rw/config/NM-system-connections/secrets/ (You can put your *.crt and *.pem files here too). Create a new file /rw/config/NM-system-connections/secrets/passwd-file.txt :

what is *.crt and *.pem mean and how to add them

Note : I am trying to install protonvpn