Last night one of my daughters asked, "what are freckles?" I did my best to explain that freckles are spots of melanin in our skin. Of course, I then had to try to explain to my five-year-old what melanin is. She then asked why she has freckles and one of her classmates doesn't. That was an answer I couldn't give beyond, "everyone's bodies are a little different." This all led to her trying to count the freckles on my arm.
After my freckle discussion with my daughter, I turned to my favorite source of kid-friendly science explanations, SciShow Kids. There I found Why Do I Have Freckles?
which does a good job of explaining what freckles are, what makes them appear, and why some people don't have any and why some people have lots of them. Should you find yourself trying to explain freckles to children, Why Do I Have Freckles?
is a good resource to consult as is the SciShow video Why Do We Get Freckles?
Applications for Education
Besides answering the question of "what are freckles?" both of these videos could be good for introducing some biology concepts to older students. At just three minutes long, both videos are a good length for making online lessons in tools like EDpuzzle or Vialogues