Let's break it down:
* Y a: This is a contraction of "il y a" which means "there is" or "there are."
* dans: This means "in."
* ton: This means "your" (masculine singular).
* cartable: This is the French word for "backpack."
Therefore, the complete phrase translates to "Is there in your backpack?"
Example:
* "Y a un stylo dans ton cartable?" (Is there a pen in your backpack?)