"The task of the educator lies in seeing that the child does not confound good with immobility and evil with activity"

Maria Montessori is revealing the idea that it is very important for educators to teach kids that being still (stable) does not constantly imply something is good, and being active doesn't always mean something is bad (evil). There might be situations where being still is not productive and being active is essential, and vice versa. The goal is for kids to discover to recognize in between what is really good or bad, despite whether it includes activity or stillness.