Just like an airplane that needs fuel, Ethereum does too. Ether can be regarded as the fuel of Ethereum. Similar to regular oil, Ether can also be bought and sold. The value of Ether also fluctuates based on the law of offer and demand. In a technical sense, Ether is the incentive provided to developers and miners to keep the Ethereum network efficient and safe. Ether is not infinite. Though it is a computer program, it cannot be replicated infinitely. So, if someone tells you that they have invested in Ethereum, it implies that they have purchased Ether.