Gas is a “fee” for every smart contract that you wish to run on Ethereum. To activate a smart contract, you need to ask Ethereum miners in the whole network to individually perform the calculations on your smart contract. Gas is a way to pay the miners of that service.
Gas is not an actual token that you can “own” even if it's a unit that can be measured. Gas exists only on the Ethereum Virtual Machine. If the smart contract is complex and requires a number and type of computational steps, memory used for storage, and many other requirements, then the more Gas is necessary to execute it. So if you need to pay Gas you will be charged with the number of ether.
Why Gas is used and not Ether directly
Similar to bitcoins, ether has a market price that is very volatile and thus the price change dramatically. Because of this, it is beneficial if there is a separation of the price of the ether vs. gas so that the cost of operations of miners will not change even if the market price of ether change frequently.
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