What is Piggy's new idea about the fire which makes Ralph feel encouraged in Lord of the Flies by William Golding?

Piggy's new revelation about the fire is that the remaining boys should keep the fire down on the beach, where they may all tend to it more easily.

This decision to move the site of the fire only comes after Jack and his hunters have defected from the tribe.  Now, with less man-power, Ralph struggled to come up with a solution.  Keeping the fire on the mountain was out of the question because of the hulking silhouette of the beast (the dead parachuter) blocking their way.

Piggy's solution is direct and simple.  The idea of keeping the fire nearby revitalizes Ralph who maintains to the boys:

"The fire's the most important thing. Without the fire, we can't be rescued" (142). 

Building the fire on the beach reinforces Ralph's desire for rescue, helping him focus on what truly matters most.