A "Leahy Amendment" is a provision in an authorization act (such as an annual Foreign Operations Authorization Act, or an annual National Defense Authorization Act) either limiting support to foreign countries or certain elements of a foreign government (such as a particular military unit) for failure to comply with human rights standards. Leahy Amendments are commonly attached to security assistance and defense authorizations.
To ensure compliance with various Leahy Amendments, the State Department conducts "human rights vetting" of governments or units the U.S. is considering providing training, equipment, or other support under authorities covered by a Leahy Amendment, prior to the support being provided.
A Latin America-focused advocacy group, The Center for International Policy, has a pretty good talking paper on Leahy Amendments.