It's a child/youth or young animal. It's not a word that is used in the real world. It sounds like something you'd bread or hear in the fantasy genre - Game of Thrones, Lord of the Rings, stuff like that.

Putting the suffix "ling" on the end of a word makes it a small thing. For example, a duckling is a small duck. As far as I know the word youngling only occurs in the star wars movies. It it a made-up word (not a real word) that should be understood as meaning both young and small. Don't use this word unless you are doing an impression of Yoda from star wars.