A ICO presale, or pre-sale, also known as Pre-ICO is a token sale event that takes place before an ICO is made available to the general public to participate.
The fundraising targets for Pre-ICOs are often lower as compared to that of the main ICO and tokens are usually sold cheaper.
A downside of an ICO presales is that early investors or adopters tend to dump tokens as soon as they become tradeable on exchanges. They often sell the tokens at the ICO price because they got the tokens for less than the price of the main ICO, making them big profits while negatively affecting the price for ICO participants.