A nestling is a very young bird that is still in the nest and cannot fly yet. Its feathers may still be wisps, and it depends on the parent birds to bring food. If you see a tiny bird that cannot hop or fly, and it belongs in a nest, that is a nestling. Though actually, English has a small family of “young bird” words, and they are not the same. A chick is a broad, everyday word for a baby bird. A nestling is specifically too young to leave the nest. A fledgling has grown flying feathers and is learning to fly — often out of the nest, still clumsy. People also stretch fledgling to mean “new and not yet skilled” (a fledgling company); nestling does not really do that job. There is another nestling that is not a bird at all: the -ing form of the verb nestle, meaning sitting or lying in a cosy way, like a village nestling in a valley. Same spelling, different build: that one is nestle + -ing, not nest + -ling. The bird is the noun you are learning here.