Question

In the above quotation, what does Paine mean by the terms "summer soldier" and "sunshine patriot"?

Answer 1

During the (first) American revolution, soldiers who joined the Continental Army during the spring or summer but went AWOL in the fall or winter were known as "summer soldiers". 

Many of the "summer soldiers" were farmers who would join up with the Army when their crops were planted, fight with them during the summer, and go back home to help with the harvest. Others would stay with the Army through the harvest, but sneak off in the middle of the night once the weather got cold. 

Meanwhile, the people who supported the revolutionaries when the revolution was going well -- but not otherwise -- were called "sunshine patriots". 

So in the famous passage from "The Crisis" where Thomas Paine wrote: 
The summer soldier and the sunshine patriot will, in this crisis, shrink from the service of their country 
he was talking -- quite literally in the former case -- about the fair-weather friends of the Revolution.