The JR Pass is a special rail pass in Japan designed to allow tourists to have unlimited travel on JR services. This includes some – but not all – of the trains and buses in Japan.
The JR Pass is useful for those who want to travel around mostly by Shinkansen (bullet train). It only makes sense to buy a JR Pass if you are going to be traveling long distances by Shinkansen.
Note that the best JR Pass for you might not cover your entire time in Japan. You might be in Japan for 14 days, but find the most cost-effective way of traveling is to buy a 7-day pass, so you should plan your trip itinerary accordingly.