Before we learn about Gas, we need to understand Ethereum. Ethereum is a software platform that performs simple computations. Those simple computations, however, happen simultaneously on a swarm of computers, called nodes.
While anyone can be a node, a special group of them, called Miners, work the hardest. They protect the network from attack and prioritize computations. Without them, there is no Ethereum. So to keep them around, they need to be paid.