"Gas" refers to the fee you pay the blockchain network for executing and storing your transactions on the blockchain. This incentivizes participants in the decentralized network to continue supporting your ability to use it!
The blockchain protocol requires that you pay a fee in order for your transaction to be accepted onto the blockchain. The fee amount will vary depending on the amount of transactions being handled by the network at that moment. This is based on supply and demand for network resources; more traffic means a higher fee. MetaMask automatically calculates the fee amount you will need to pay in order to get your transaction through.
In some cases you might wish to manually adjust the gas fee. Reasons might include the following:
You want your transaction to be processed at a different priority (slower or faster than normal)
Your wallet isn't updating the gas price
If you want to adjust the gas price manually, click EDIT. When manually adjusting the gas price, it is recommended that you use a gas tracker to check the current price. It will help you decide what price to set based on an estimate.