1999 Toyota Camry Starter and Flywhee;

I have already replaced the starter, about 18 months ago. Have been having problems with the starter for over a year. Sometimes it happens only once every few weeks and sometimes every other day. Try to start the car and get a grinding sound, try it again and the same thing happens and the third time it starts.
Just took it in to have it replaced and was told that I also needed a flywheel.
What exactly is the flywheel and how does it affect the starter.