An ICO, the abbreviation of Initial Coin Offering, is a term that refers to cryptocurrency companies selling their tokens to raise funds. You can compare buying tokens through an ICO to buying shares in an Initial Public Offering or IPO.
However, one big difference is that the IPO market for shares is highly regulated while the ICO market has been mainly unregulated, and has therefore experienced some fraudulent ICOs and scams.
Sven Franssen