Gas & Gas Limit
Gas measures the computational work a transaction requires; the gas limit is the maximum you are willing to spend on it.
In depth
Every operation the EVM performs has a gas cost, and you pay gas × gas price in ETH. A simple transfer uses about 21,000 gas; a complex DeFi trade may use hundreds of thousands. Setting the limit too low makes the transaction fail (and you still pay for the work done).
Why it matters
Gas explains why Ethereum fees vary so much and why failed transactions still cost money.