ICO stands for Initial Coin Offering. It is a form of crowdfunding, used by startups.
Imagine a team of developers who are working on a new promising project. They might need to raise money from public, so they create their own cryptocurrency. Those who want to support the project, would buy the rights of token possession during the initial crowdsale phase. As a result, the project gets the capital to fund development, and the audience members get their tokens’ shares. This whole process is called ICO.
It’s very important to understand that during the initial phase new coins don’t exist yet, people are buying IOU (“i owe you”) – not the coins themselves, but the obligations. As soon as fundraising phase is over, IOUs are being transformed into usual tokens.