She remembers coming home from a computer science class in tears, having stayed up all night to get a program to work. It didn’t. “My dad said to me,” Estrin recounted in a 2010 interview, “that the key to solving problems in programming, which I think applies to life also, is first to look at that big problem and break it into pieces.” Figure out how to solve the smaller problems and then figure out how they work together.
A Women’s History of Silicon Valley
Jessi Hempel

This is literally the short answer I give when someone asks me to explain what programming/computer science is.