The ‘light user’ can be seen as an end-user that uses your application from time to time, like: every month 1 or 2 times. Whenever the end-user needs to be there.
The ‘regular user’ can be seen as an end-user that uses your application on a daily basis. This end-user’s core job exists of using the application frequently. The usage is way higher then a light user.
Hope this clarifies things for you.