It’s a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet.

BGP is classified as a path-vector routing protocol, and it makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator.

To quote Cloudflare “Border Gateway Protocol (BGP) is the postal service of the Internet. When someone drops a letter into a mailbox, the Postal Service processes that piece of mail and chooses a fast, efficient route to deliver that letter to its recipient.”