It's a question asking for a description of girls in general, like their personality, appearance, or behavior.
Here are some examples of how you could use it in a conversation:
* "¿Como son las chicas en tu escuela?" (What are the girls like at your school?)
* "Me preguntaba como son las chicas de esa ciudad." (I was wondering what the girls from that city are like.)
Let me know if you have other Spanish phrases you'd like me to translate!