Gas

Note that fees are still paid in ether (ETH), but gas and ETH are two different things. Simply put, computational tasks are measured in terms of “gas cost”. On the other hand, each unit of gas has a “gas price” that is defined in ether (ETH). As such, every transaction has a particular “gas price” for each unit of gas. 

So if you need your transaction to be validated quickly, it makes sense to pay higher gas prices, so that validators are incentivized to verify your transaction first than others. Similarly, setting a low gas price can cause your transaction to be stuck as validators won’t have any incentive to validate it.

The gas pricing mechanism is important because it guarantees that fees are being charged in a fair and appropriate way. So it prevents resources from being wasted on operations that are not valuable to the Ethereum network.