One way to look at Blockchain technology is as a currency of computations. In the analog world, payment is made for physical labor, while in the Blockchain world, payment is made for digital work. For every transaction or interaction with a Smart Contract on the Ethereum blockchain, a small (ETH) gas fee is charged to the user for that computation, and it is paid to the digital miner who mined that block (or made that computation). Extensive measures are made by the community to ensure that gas prices never get too high, and individual ETH owners can choose to limit how much gas they are willing to pay. To watch real-time ETH gas data, check out this cool app ETH Gas Station.