It's hard to explain from scratch.
Caddy is a reverse proxy software that essentially redirects traffic from a certain port to another port. For example external:port => internal:port. It also enables SSL encryption meaning everything will be encrypted en route between the external and the user.
VPS is a virtual private server. Just someone else's computer you can expose to the Internet.
Tailscale is a mesh VPN that uses wire guard as its transport. I use this to tunnel between my VPS and my Immich server to hide my home IP and to allow encrypted traffic between my Immich server and my VPS.
A zero-day (also known as a 0-day) is a vulnerability in software or hardware that is typically unknown to the vendor and for which no patch or other fix is available. The vendor thus has zero days to prepare a patch, as the vulnerability has already been described or exploited.
There's no fix other than security through layers.
I want to be able to upload/download/share my photos from anywhere in the world without using a VPN. Additionally, this satisfies the wife requirement. It works in the background without her needing her to turn on the VPN. I don't want her to keep asking me how do I turn on the VPN? If it's just me, then no issue, I'll use a VPN.