Keystone habits were described in The Power of Habit by Charles Duhigg. The idea is that instead of a changing the big habits in life we should focus on small seemingly inconsequential habits and work towards changing them. The result will be an easier change in the bigger habits.
Does this approach work?
That book is pretty much the only popular reference to the concept of Keystone Habits. It does not seem to be present in any other popular books/sites.