The English term 'ratify' means approve and express assent, responsibility, or obligation
Simply, it means to approve. It generally refers to some sort of large governing body approving of something such as a bill, an econimc motion, or something as drastic as a new system of government.
Verb: Sign or give formal consent to a (treaty, contract, or agreement), making it officially valid.
Means approving it
To ratify the U.S. Constitution means to formally approve the Constitution.