A hatchling is something that recently hatched, like a baby bird or lizard. The plural works just like you'd expect - it indicates that there are multiple hatchlings.

"Hatchlings" are baby birds that have just come out of their eggs. In English, the verb for a bird to come out of its egg is "to hatch". Informally, it can also be used to talk about someone that is very, very new at doing something, such as driving a car. But, this isn't very common. 99% of the time, you're talking about a baby bird.

It usually refers to newly-hatched baby birds.

Chicks that have just come out of their eggs, and can't fly yet. They become 'fledglings' as soon as they can fly.