To ratify a treaty or contract is to officially approve it by signing or voting for it. You and your brothers and sisters might devise a plan for a family vacation to Disney World, but it would need to be ratified by your parents.
You are most likely to hear the word ratify when talking about laws. In the U.S., Congress writes bills, but they need to be ratified before they become law. An amendment to the U.S. Constitution must be ratified by three-fourths of the states, either passed by the state legislatures or by state conventions.