The sentence "Health education is a continuous process and not a one-shot mechanism" means that:
Health education must be given time and again and periodically, and not once. The knowledge, behavior, and needs of people change with the passage of time, and therefore they need continuous information, reminders, and motivation in the form of creating and nurturing good habits. One single session or message will not yield sustainable change.
In short: Health education has to happen over and over in order to work.