Well, it can mean a number of things, but generally speaking you can think of it as an interface (hardware and software) that allows one network to connect to another network for the purpose of transmitting information. This is referred to as "internetworking." Computer networks use gateway servers, bridges, and routers to provide this function. Within telecomm the devices are technically different and are generically referred to as Network-to-Network Interfaces (NNI). Physically, they are adapter cards sitting in a rack in a telecom cabinet.
An example that comes to mind is the Internet access gateway many mobile phone service providers employ in their networks to allow subscribers to use text messaging (SMS and MMS) to send messages to email addresses on the Internet. You include a gateway code in your message header, and that tells the SMS/MMS server that instead of sending the message to another cell phone, route it through the access gateway to the Internet.
I'm sure their are many other examples (Wi-Fi networks interfacing to the Internet, etc.), but that is one I have recently investigated. Hope this helps.