A gateway sits between devices that speak a local protocol and the network or cloud that speaks IP. A LoRaWAN gateway receives radio packets from sensors kilometers away and forwards them over Ethernet or cellular; a Zigbee hub turns mesh traffic into Wi-Fi; an industrial gateway reads Modbus registers from a PLC and publishes them over MQTT.
Beyond translation, gateways are where local intelligence tends to live. They can buffer data during an outage, aggregate readings before sending, enforce security policies and run analytics close to the source, at which point people call them an edge gateway. The trade-off is a single point of failure and another device to patch, so larger sites often deploy gateways in pairs. See Edge computing explained for the processing side.
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