In case of an ICO, there are several stages that the project goes through, in terms of collecting funds from investors.
To make sure that the funding will be a success and to attract large investors, ICO's offer the option to engage in a so called pre-sale. This is usually only open to investors willing to invest more than certain amount and there are bonuses prepared for such investors. The most used bonus is an extra amount of tokens on top of the agreed amount, which is basically a discount from the original token price.
Large part of well executed and promising ICO's reach their Hard Cap during the pre-sale phase already.