A soft cap is the minimum needed for the project to proceed and a hard cap is the amount that they will accept no more than. For instance a soft cap could be 10 million and a hard cap 30 million meaning if the project does not attain 10 million it will not proceed and also if it reaches 30 million no more can be contributed.
And expect a huge bonus when you invested on that ICO or participated in its bounties. Campaigns usually are finished early also whenever the ICO bounty campaign reaches its desired hardcap.