Foederati

Foederati is a big word that means people who are not originally from a country but have made an agreement to work and live there peacefully. Imagine you and your friend who are not from your neighborhood want to go play in your neighbor's yard. Your neighbor may not want you playing there, but if you promise to play nicely and not disturb anything, your neighbor might let you play there as foederati. This is like how people from different countries may agree to live peacefully and work together in another country, like ancient Rome. They made an agreement with Rome to work together, and in return, Rome protected them from other enemies. This helped Rome expand its power and influence.