Ethereum Gas

Gas is the internal pricing for running a transaction or contract in Ethereum. It's purpose is to decouple the unit of Ether (ETH) and its market value from the unit to measure computational use (gas).

Ethereum miners are free to ignore transactions whose gas price limit is too low...If there is not enough Ether in the account to perform the transaction or message then it is considered invalid. The idea is to stop denial of service attacks from infinite loops, encourage efficiency in the code – and to make an attacker pay for the resources they use, from bandwidth through to CPU calculations through to storage.

The more complex the commands you wish to execute, the more gas (and Ether) you have to pay.¹

[1] https://www.cryptocompare.com/coins/guides/what-is-the-gas-in-ethereum/