A hard cap is the maximum amount of money that a crowdsale (e.g., an ICO) will receive. It is set by the project leaders and it is commonly tied to the expected tokens supply.
A hard cap is often a good way for an investor to determine the importance of a project even if it is not an exact science. The hard cap can eventually be set low and create a significant increase of the tokens’ value post-ICO.
See: soft cap.