i hear this term often too, i guess its like an update of version like computer ?
A hard fork is a software upgrade that introduces a new rule to the network that isn't compatible with the older software. So, those who were still running the old version of the software will see the new transactions as invalid.
Yes you have explained very well actually it is used for introducing the upcoming new rules where there is no compatibility between the new one and older on softwares which help you in knowing the validity of your transactions either these are valid if new one while invalid in case of running the old version of your software.