ICO, or Initial Coin Offering (primary placement of tokens) is a mechanism for raising funds, under the terms of which future crypto currency is sold for current, liquid virtual money. You list ICO bitcoins or ethers, and in return receive tokens of the new project. This is another realization of the crowdfinding model, when participants finance the development of the company now in order to get some good from it in the future.
PreICO is the stage preceding the ICO (main stage) of raising funds for the project. PreICO is conducted to test the demand for the project, as well as to check and get support from the community, to expand the budget for marketing before the main ICO. Not every project carries out preICO.