Initial Coin Offering is the Crypto version of an IPO (initial public offering) ... in the world of Wall Street, companies emit securities (stocks) and sell them to investors who show interest, that's how the companies get funding.
With an ICO, the principle is the same ... company announces a project that utilizes a new token/coin and offers to sell this new token to investors, to secure funding for the development of their project.
Most of the ICOs are executed on the Ethereum platform, so the ETH currency is used as a form of payment for the newly issued tokens. The rising popularity of ICOs also caused the huge popularity of Ethereum.