Complex sentence: Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco that can encapsulate a wide variety of network layer protocol packet types inside IP tunnels, creating a virtual point-to-point link to Cisco routers at remote points over an IP internetwork.
Split into simpler sentences: Generic Routing Encapsulation (GRE) is a tunneling protocol designed to encapsulate network layer packets. GRE creates a virtual point-to-point link with routers at remote points on an IP internetwork.