https://info.etherscan.com/what-is-ether-eth/

Ether, or ETH for short, is a digital token minted in the Ethereum network that acts as the network’s main currency. It is oftentimes used by users as a store of value and a method of payment. Aside from that, it also has other notable uses:

• To pay gas fee (What Is Gas Fee?)
• As mining reward

Ethereum is an open-source, decentralized world-computer network that is not owned, supervised or controlled by any singular central authority but that does not mean that it is free to use.

When we engage with applications on the Ethereum network, this action will be recorded as a transaction which will later be verified by miners before it is permanently recorded into the blockchain. These processes consume time and computational resources. If transactions are free, users could flood the network with spam transactions and eventually causes congestion that slows down the entire network. So, it is ingrained in its nature that every transaction inside the Ethereum network will incur a transaction fee (or exclusively known as gas fee in the Ethereum network), payable with Ether.

A gas fee is paid by the sender in every transaction and will usually affect the speed in which the transaction is processed by the miners. A higher gas fee would usually mean a faster transaction speed, especially during times when the network is congested with pending transactions.

Since Ethereum is a decentralized network, this means that there are no designated staff or employees hired by a central authority behind desks making sure all the operations within the network are going well and as planned. Hence, an incentivisation system is introduced which requires and promotes computational contributions from the network users, through which these users will be rewarded in the form of ETH. These users are called miners, and what they do is contributing computational power by verifying the transactions in the network - this verification process is called mining.

In short, ETH is essentially the fuel for the Ethereum network that keeps it running and decentralised, without which will cripple the essence of Ethereum altogether.