Helium is a global, distributed network of Hotspots that create public, long-range wireless coverage for LoRaWAN-enabled IoT devices. Hotspots produce and are compensated in HNT, the native cryptocurrency of the Helium blockchain. The Helium blockchain is a new, open source, public blockchain created entirely to incentivize the creation of physical, decentralized wireless networks. Today, the Helium blockchain, and its hundreds of thousands of Hotspots, provide access to the largest LoRaWAN Network in the world.
Here is a great video to explain what Helium is and how it works.