A gas unit is the smallest type of work that is processed on the Ethereum network.
Validating and confirming transactions on the Ethereum blockchain requires a certain amount of gas, depending on the size and type of each transaction.
Gas is the unit of measure for how much computational work is required to process transactions and smart contracts in the Ethereum Virtual Machine. More complex smart contracts, and code, will require more gas to execute. (Just like how bigger and/or faster cars will require more gas to power them).
Gas limit is the maximum amount of gas you are willing to spend. when you are sending ETH/transactions, you will be required to specify gas limit and gas price
Related read : How to customize gas price/Limit