Border Gateway Protocol - routing protocol used to exchange routing information between different networks on the internet. Like having traffic coordinators that help data find the best routes across the global internet.
Internet service providers use BGP to determine the best paths for routing data between different networks worldwide.
BGP is a standard routing protocol (not a cloud service). Cloud providers commonly use BGP on dedicated/private connectivity (Direct Connect/ExpressRoute/Interconnect/FastConnect) and on managed routing components (e.g., GCP Cloud Router, OCI DRG) to dynamically exchange routes between your network and the cloud.