BGP Border Gateway Protocol
The protocol routers on the internet use to tell each other "I can reach these IP prefixes, route through me".
The internet isn't one giant router but thousands of "Autonomous Systems" (AS) that peer with each other. BGP is how they negotiate the path between them.
Famously vulnerable to misconfiguration — when Facebook went down in 2021 it was BGP-related. A BGP leak from one operator can take down large parts of the internet.