It is the comparative form of the adjective "jeune" (young). It is used to compare the age of two people or things, indicating that one is younger than the other.
Here are some examples:
* Il est plus jeune que moi. (He is younger than me.)
* La fille est plus jeune que le garçon. (The girl is younger than the boy.)
* Cette voiture est plus jeune que la mienne. (This car is younger than mine.)