The gas fee is a fee that developers have to pay to the Ethereum network in order to use the system.
When you send a letter with a mail carrier, you pay a postage fee.
If you want your letter to get there sooner, you pay a higher fee.
During peak times, because of the high volume of letters being processed, it takes more time for the carrier to deliver them all. In that situation, the carrier then prioritizes those letters that paid the higher fee to get there sooner.
This is similar to how gas fees (aka transaction fees) work. There is a gas fee for every transaction made on most blockchains. If the blockchain network is busy, you pay a higher gas fee for your transaction.
Now, for all of you nerds out there, check the link below for a more in depth description of what the gas fee is.
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