ICO stands for Initial Coin Offering. It is a fundraising method used by cryptocurrency projects to raise capital by selling their newly created tokens to investors in exchange for established cryptocurrencies like Bitcoin or Ethereum, or sometimes even fiat currency.

During an ICO, the project team typically creates a whitepaper that outlines their business model, project goals, and details about the token being offered. Investors interested in the project can then contribute funds to the ICO and receive the project's tokens in return. These tokens can represent various things such as utility within a specific platform, voting rights, or even potential future profit sharing.

ICOs gained significant popularity during the cryptocurrency boom in 2017 and 2018. Many blockchain-based startups utilized ICOs as a means to secure funding for their projects without the need for traditional venture capital investments. However, it's important to note that ICOs can carry certain risks, including regulatory concerns, lack of transparency, and potential scams or fraudulent projects.

Following the surge in popularity, regulatory bodies in many countries started paying closer attention to ICOs, leading to increased scrutiny and the development of guidelines and regulations to protect investors. As a result, the popularity of ICOs has somewhat diminished, and alternative fundraising methods such as Security Token Offerings (STOs) and Initial Exchange Offerings (IEOs) have gained traction.

Overall, ICOs played a significant role in the early stages of the cryptocurrency industry, providing a means for startups to raise funds and investors to participate in promising blockchain projects. However, caution and thorough research are necessary when considering participation in an ICO due to the associated risks and potential legal implications.