Hardfork = a modification in the underlying logic of the blockchain, impacting the way transactions are made from a certain block onwards.

Starting with HF19, for instance, the voting algorithm will be modified so each vote will have a proportional saying in drawing rewards from the reward pool (as of now, this is is squared, so if you have n voting power, you draw n^2 rewards from the pool).

Think of a hardfork as a new version of the blockchain, from a certain moment onwards.