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 routing protocol (not a standalone cloud service). All major clouds use BGP to exchange routes for hybrid connectivity (private circuits and VPNs) and for dynamic routing between on-prem networks and cloud virtual networks.