each time you turn your ignition key, the starter gear must engage with flywheel teeth, so the starter can crank the engine. if they do not engage very well, the grinding happens.
to solve the problem you need to change the starter and the flywheel if the teeth of flywheel is worn out. this costs lot of money especially for flywheel changing.
the temporally solution is to let starter get as much current as it can when you try start your car, so the starter can push its pinion gear harder to engage with flywheel teeth.
so check your battery. if it is more than five years old, chagne it. clean the battery terminals. check the battery cable, if it is cracked, change it (it is not an easy job). another modification is to let starter solenoid get its current directlly from battery, not from ignition switch.