On Ethereum, gas units serve as a proxy measure of the computational resources required for transactions. Each transaction consumes varying amounts of bandwidth, storage, processing power, and memory from network participants in its relay, validation and settlement, and so in order to understand a transaction’s resource cost, the Ethereum protocol created a representative unit called “gas”.
More complex and resource-heavy transactions naturally have a higher gas cost, representing the increased resource load such transactions impart on the network.
The number of transactions the Ethereum network is able to process is limited by the number of gas units available in each block. With this design, the Ethereum protocol sets an expectation of how much real-world resources its participants will need to dedicate to network operations. Also, by limiting the total number of gas units available over time, the Ethereum community is limiting the cost of its participants operating the network, which is an intentional design choice meant to support the overall project goal of encouraging as large and widespread of a network as possible.