This is a true statement. When we observe something in the scientific world over a long period of time and the same observation is made repeatedly without variation during that time, then it is a scientific law. If I hold my arm out and drop a ball one hundred times in a row, and each time it falls straight to the ground, I can say that is a scientific law.
I wouldn't try to explain why that happens each time, I would simply summarize the observed reaction. Hope that helps!