An ICO (Initial Coin Offering) is a public sale in the world of cryptocurrency.
When a project wants to launch its own cryptocurrency or DApp, it can open an ICO to attract investors to its ecosystem.
ICOs are basically a crowdsale on blockchain, i.e. the crypto version of crowdfunding. So anyone can invest in a project in which they are interested by buying its tokens and consequently becoming part of that project.
A historical ICO was Ethereum’s in 2014, which is now the most widely used platform to launch ICOs on.