When people settle down, they start to live a less active life and perhaps get married, buy a house, and start a family.

1. When you settle down, you become less active, nervous, or upset. When you settle other people down, you do something to make them less active, nervous, or upset. Calm down is similar to settle down.
2. When a confused or violent situation becomes less confused or violent, it settles down. When you settle a confused or violent situation down, you make it less confused or violent and more calm. Calm down is similar to settle down.