I’m not sure to understand what makes it different from WireGuard. Could someone eli5 ?

Vanilla WireGuard doesn't provide a way to run a peer in userspace that can proxy traffic between another peer and an endpoint such as a web server because you need to be privileged to do things like work with raw packets. However, https://github.com/WireGuard/wireguard-go is a userspace implementation of WireGuard and has recently incorporated Google's userspace networking stack. This project uses these two userspace tools to "fake" a privileged WireGuard peer that proxies TCP, UDP, and (a small subset of) ICMP. It was written as a pentesting/red team utility for my team but it can also serve as a general makeshift VPN when you don't have privileges on a box you want to proxy through.

Edit: typo