Initial Coin Offering. It is when a project sells off a certain percentage of their coins to help fund development and give the project runway time to develop and bring their app/product/platform/whatever to market.
ICOs are especially risky because not only are you buying into a new cryptocurrency, but there may not already be any existing product or market for it, it could very end up being vaporware, a scam, or it could just legitimately fail despite the development teams best efforts.
It is supposed to be the crypto-equivalent to an IPO (initial public offering) of company stock, but in the crypto world there are much less (if any?) regulations to protect investors. Definitely do your own research and make sure you understand what you are buying and what you are investing in before investing anything into an ICO.