Looking for a Blockchain Expert who can Answer Questions About Setting Gas Price and Units on Ethereum Transactions

I am writing a layperson’s introductory investment series on CryptoKitties, which uses the digital wallet MetaMask to process transactions. Gas price and units are both variable, so I’m looking for an expert to explain what gas is, why you must pay for each transaction you make and what it means to adjust gas price and units. My goal is to help the reader understand how gas affects Ethereum transactions and the cost of playing CryptoKitties.