"You are going to settle down." means:
You're going to start a stable, long-term life, often by getting married, starting a family, or establishing a permanent home and career. Examples:
◇ "After years of traveling, you're finally going to settle down."
◇:"Do you think you'll settle down in Addis Ababa or move somewhere else?"
It can also simply mean to become calmer and more settled in life, depending on the context.