Well unless you're referring to something else, a hardfork with respect to cryptocurrency and the Blockchain as a whole is a complete change to the protocol that at the end of the day makes previously invalid transactions or "blocks" valid and vice versa. This is done by making all users/ switch over to the latest version of the protocol software. It is basically a permanent deviation from the last version of that particular Blockchain and as such nodes/users which are still using the former version won't be accepted by the new one. This results in a sort of split or "fork" in the Blockchain in which one one path follows the new protocol while the other follows the old one. Eventually those using the old version or the old Blockchain protocol switch over to the new one when they realise that the old one is obsolete.