When you send tokens, interact with a contract, send ETH, or do anything else on the Ethereum blockchain, you must pay for that computation. That payment is calculated in Gas and gas is paid in ETH.
You can see your TX fee (gas limit * gas price) in ETH & USD when you search for your transaction on etherscan.io. This fee is paid to miners for mining transactions, putting them into blocks, and securing the blockchain.
You are paying for the computation, regardless of whether your transaction succeeds or fails. Even if it fails, the miners must validate and execute your transaction (compute) and therefore you must pay for that computation just like you would pay for a successful transaction.
The total cost of a transaction (the "TX fee") is the Gas Limit * Gas Price.
You can think of the gas limit like the amount of liters/gallons/units of gas for a car. You can think of the gas price as the cost of that liter/gallon/unit of gas.
With a car, it's $2.50 (price) per gallon (unit).
With Ethereum, it's 20 GWEI (price) per gas (unit).
To fill up your "tank", it takes...
10 gallons at $2.50 = $25
21000 units of gas at 20 GWEI = 0.00042 ETH.
Therefore, the total TX fee will be 0.00042 Ether.