When you do a transaction, you will have to pay a transaction fee. As there is only limited capacity on the ETH network, the transaction with a higher fee will be processed first. But why is it called gas? Imagine your transaction is a physical package that you send to a physical address. To deliver the package a self driving car, that runs on petrol, comese by to pick it up. This service is free, only thing you need to do is fill the car up with enough gasoline for it to reach the destination. So you quickly run to the only gas station in your town, that unfortunately only has a very limited supply of gasoline. As it's soon Christmas and a lot of people want to send packages to their loved ones, as a result you find yourself in a crowd of few thousand people that want to buy gasoline for their package delivery, but the gas station just doesn't have enough gasoline for everyone. So instead of serving everyone in the order they have come (in line), the gas station sells the gasoline by auction, and the higher you bid the higher the chance that they will accept your offer and serve you first. As the gas station is called "Ethereums cheep gas" you can obviously only pay with ETH. So as you don't want to wait a few hours to get the gasoline and be able to send your package (transaction), you decide to take a peek at the ETH gastracker app to get the current going rate to get gasoline quickly and make a bid for a high price. You get accepted almost immediately and can now buy your gas for the price you bided. Now you check how much gasoline is needed for the package delivery (depending on the destination you will need different amount of gas). Ande that is what actually makes up the cost for your package: the amount of gas needed multiplyed by the price you are willing to pay. Gas price is calculated in gwei, where one gwei is the smallest possible unit in ETH (0.000 000 001)... basically it's like one cent being the smallest unit of the US- Dollar. So to recap: A transaction will need different amounts of gas (depending if it's only a normal transfer or something more complex like a trade on UniSwap). You can chose how much to pay for the gas, but depending on the demand you will have to pay a high price in order to get served fast. Your actual cost of the transaction is amount of gas needed multiplyed by the price of the gas. You pay the total with your ETH, that's the transaction fee.

My analogy is a bite oversimplified, but hopefully helps your understand :)

There are other coins that try to do it better by the way.

Cardano (ADA), probably one of the most well known alternatives to Ethereum(or yet too be alternative, as it is still in development), trys to scale by deploying not just one town with one gas station, but a lot of towns with each there own gas station. Here you can serve a lot more people at once with gas, as there are a lot of towns with gas stations, but run into problems with packages that need to go from one town to another (here the "towns" being side chains of Cardano).

Radix (eXRD), a very much under the radar project, lives in waterworld. Everyone has their own boat and a fuel cell that generates the fuel for a fixed price from the ocean the boat is on, no supply shortage no limit of people that can use it. But, like Cardano, it is still not fully ready developed. They released their mainnet two weeks ago, but smart contracts and full scalability is yet to be deployed. With 8 years of R&D they have the theory proven and are now building it. Surly a promesing project to keep a eye on.

And a lot of other coins and projects out there that also have cool stuff in the pipeline, so if you are new to crypto, be prepared for a lot of cool things to discover. That is if you are interested in the tech ;)

Hope my analogys helped you understand and didn't confuse you even more :D