What's new and green? That'd be the new greenery you see on trees and other plants every year!

新緑 refers to new foliage — fresh green leaves of late spring and early summer, just after they've fully emerged. In Japan, it's seen as a symbol of that time of year, so it often carries a slightly poetic nuance.

This is a jukugo word, which usually means on'yomi readings from the kanji. You've seen them before (remember the りょく reading from 緑茶?), so you should be good to go here.