ICO is short for Initial Coin Offering, it is derived from the term IPO (Initial Public Offering).
At the time of the initial release, the issuer can choose to sell their crypto for a limited time. The process of this limited sale is called ICO.
But many of the current ICOs are scams, so if you are interested in ICO, then you must be careful.
In my opinion, the safest way to invest in crypto is to buy bitcoin. The risk of ICO is very high.