The proverb "Rome wasn't built in a day" means that great things take time and cannot be accomplished overnight. It is a reminder that patience and perseverance are necessary for achieving any significant goal or completing any complex task. The expression also implies that the process of building something worthwhile is slow and requires a lot of hard work, dedication, and attention to detail. It is a timeless piece of advice that encourages people to be realistic about their expectations and to stay focused on their long-term objectives.