A gateway is a small piece of hardware or even just software based that leverages network connections. It essentially is a network node that acts as an entry door to another network.
This interconnecting system also has the capability to link those networks even if the networks are working under different standards.
Gateways are usually deployed at the edge of the network and their importance and use is increasing at speedy rates as the IoT requires more edge computing and analysis than we have experienced ever before.